Firearms against low-tech enemies, especially in II. Combat is divided into two phases: fire and shock. Before you gain fire value for any of unit types, fire phase is skipped for your armies. Fire phase by default mostly affects morale. So for half the combat, low-tech armies are doing nothing, while getting their morale wrecked.
From III onward, any tech or culture group that gains fire rating as a first will steamroll neighbours who will need few additional techs to gain fire rating on their own. This is especially true for Ottoman tech group, which not only starts ahead technologically over anyone who isn't Ottoman or Arab, but also is the first to gain fire PIPs on its infantry and intentionally sonote This all but guarantees historical conquest of Balkans, where most countries are of Eastern tech group, so not only they research at slower rate than Western Europe, but their first infantry armed with firearms won't show for few decades more than Western counterpart. Even when IV reworked research and removed tech groups, the culture group stayed, so Eastern infantry is still locked away from wide use of firearms early on.
Artillery in all parts, especially early on. It can turn sieges into relatively short affairs, since the scaling of artillery-to-fort ratio is pretty low at this point and it will take AI quite a while to fortify adequately all provinces. And that's assuming AI has tech to improve forts in the first place. Starting from III, artillery can double your combat power, as it gets positioned in the second row of fighting armies. Since AI can't really compose proper armies nor utilise artillery at full efficiency, it can quickly turn wars into series of curb-stomp battles. Especially since artillery is not affected by culture group and everyone gets the same guns with tech progression.
Since most of battles are resolved by morale drop and route rather than troops being wiped out, combination of few, even minor morale bonuses can make a massive difference, especially at low tech. There is also "sustained morale" in II and III, which represented overall morale of each side of the combat. By throwing even minor, but fresh troops into the grinder in timed intervals, it was possible to keep morale high, eventually defeating even superior forces. IV changed the combat resolution mechanics to decrease efficiency of this exploit, but it's still there.
Latin tech group in II is the only one without any research penalties. Due to the innate penalties of other tech groups, even a very skilled player will have a hard time keeping up with even most backward country from Western Europe if they decide to play as, say, Mughal Empire. And since countries within the same tech group gain a "catch-up" bonus toward the current technological leader for free, all it really takes is a single strong Latin country for everyone else in Western Europe to just band-wagon. III toned it down, adding more intermediate groups and ability to reform your government and academia for more efficient ways and IV first replicated the mechanics from III, then removed the tech groups entirely and replaced them with institutions.
Europa Universalis II
The so-called tiny armies. Due to the way how rounding was done, army consisting of just 1 soldier was pretty much immune to attrition, making them perfect for exploration and to keep provinces on permanent occupation without tying down any troops. They were also perfectly suited for abusing retreat pathfinding and synchronised looting. Of course getting to that 1 soldier required an absurd amount of splitting and dividing of your troops, but the few minutes spent on it were well worth the effort. Just don't expect such army to put out a fight.
There is a highly abusable mechanic of exchanging maps and charters via diplomacy. In practical terms, at the cost of 2-3 diplomats and a single loan, any country from Europe could gain maps of Middle East, Persia and most of India as early as February 1419, opening local markets far ahead of time. Do it as any of the historical trade republics, who start with trade tech advanced enough to gain monopolies (not to mention ability to out-compete other merchants) and you can gain obscene amounts of money. Rinse and repeat to gain access to the Malacca and then Chinese markets. This was breaking the game on so many different levels, Paradox simply cut the ability to swap maps from III onward.
Monopolies, due to snowballing effect they have. Aside income alone from 6 merchants and all the possible free slots in a Center of Trade (which can go as far as taking income from all 20 slots, so the entire trade value of the CoT), they provide a passive benefit in form of +1 merchant per year and +2% trade efficiency. Per monopoly. And it's relatively easy to maintain around 10 monopolies by mid-game. The trade efficiency translates into how much trade income you get from all those monopolies and it doesn't have a top cap, so you can potentially end up with a monopoly generating higher income than the trade value of that particular CoT. With the bonus merchants, it's piss easy to just keep going and gaining further monopolies or maintaining the existing ones. Even a single monopoly achieved for a month is worth it due to the 2% income of all your trade thanks to that single monopoly. Just like exchange of maps, this was so ungodly powerful, Paradox cut the trade efficiency bonus in III and reworked monopolies into Cool, but Inefficient due to everyone competing with the current monopolist.
The bigger the fleet, the less affected by attrition it is. A fleet of 101 ships is completely immune to supply attrition, regardless of tech level, meaning it can stay on the sea forever without suffering any loses due to attrition. In more practical terms, a fleet of at least 17 ships will never be automatically ordered to return port after hitting 6% of naval attrition (about 4 months at sea in earlier stages of the game, where it matters most). Said return is usually what kills the fleet, since it will spend another few months sailing back to closest port, often picking open ocean, further increasing attrition and loses, rather than just staying to shoreline, where it is safe from attrition. Using this knowledge allows to explore the world in much, much more efficient way, while decreasing loses to attrition to minimum.
Castille and Spain can gain access to Columbus in 1486, when he approached the crown for the first time with his plans for expedition. Since non-random explorers last until their historical death, this means gaining 6 extra years out of Columbus' service for no downside and all the benefits of exploring Americas and African coast for additional six years. It is perfectly possible to circumnavigate South America or instead reach the real Indies (by going around Africa) with use of that time, while establishing colonies along the way.
Correspondingly, there is nothing preventing Portugese from sending their explorers, intended for African circumnavigation, to find Brazil instead, some 70 years ahead of schelude. And it's only three sea zones away from Capo Verde, making it an easy task from the get-go. Bonus point for Brasil having half of all tobacco provinces in the game, so that small deviation from course is going to pay back handsomly in Tago's CoT.
Calvinism (or 'Reformed', as the game refers to it internally) is by far the strongest pick of all the religions. It offers a 10% trade efficiency bonus (i.e., 10% more income from all trade, which is where the lion's share of income comes from) and a -4% discount on technology. It is pretty rare, too, so there is a very low chance of ever facing a stability hit for declaring war on a country sharing the same faith. It also has the same number of colonists as Catholicism, allowing for the building of a solid colonial empire, while ignoring the effects of the Treaty of Tordesillas, because it's obviously not Catholicism. The only downside is that due to the way religion, conversion and the ability to convert work in II, you either play in regions historically following Calvinism (like the Netherlands, that really shine when following this religion) or you will never be able to benefit from it or even convert to it with simple missionary work.
Europa Universalis III
There used to be a spy mission giving you a core on a neighbouring province. This not only justified conquest of said province (or provinces), but removed the problems with nationalism-based revolts once conquered and provided full tax income right from the start, rather than waiting for 50 years before core generates on its own. Yeah. Paradox quickly cut that one out, for obvious reasons.
Due to changes in how colonisation works, from In Nomine onward, all player has to do is settle a province once. If colonial maintenance slider is set to maximum, the colony will achieve 1000 population (and thus turn into a city) in 18 years. And only 12 if Land of opportunity national idea is taken. This was done to change dynamics of colonisation. In previous versions of the game, the goal was to have as high as possible yearly gain of colonists and simply keep sending them. In Nomine greatly reduced colonist growth, but in turn increased growth of colonial population by the factor of 10. While it was done to make colonisation less of a hassle and provide AI with ability to build colonial empires more easily, in reality it allowed to absolutely spam colonies, especially if you just started exploring. The pre-existing stock of 5 colonists allowed to take 7 different provinces in the very first year of finding new lands and then steadily keep adding 2 more each year.
Before the game starts, one of the setting is how much population increases with each colonist send to the province. Set to maximum and colonies turn into cities within a year. Of course this is a double-edged sword, since the setting affects everyone, but there is still exploration itself and colonial range to factor into it. And AI is incompetent in managing both of those, not to mention anyone who isn't Spain and Portugal will have insufficient range to even try. If you are playing as any of the Iberian countries, you can easily block colonisation by other countries, simply by taking over all the provinces that would eventually be in their range, thus not giving them staging ground for further colonisation. And if natives in Americas start colonising on their own (which is rare, but happens), this makes conquest of their lands all that more valuable, since there are more provinces to be swallowed.
Unlike II, where CoTs are pre-generated and automatically generate new one should there be suitable province for that or IV, where entire trade system is set in stone no matter what, Centers of Trade in III can be build. This allows to generate "extra" CoTs in freshly colonised areas or conquered footholds into certain regions, completely shifting the trade, not to mention placing them in most beneficial provinces. And if your country is mercantilistic (higher chance of merchant placement in CoT within country's border), this in turn allows to easily monopolise trade in given CoT and then maintain said monopoly. This also allows to solve the over-competition in various huge CoTs with numerous OPMs in the area - rather than having a single big market where everyone is fighting for a spot, open one (or even two) more and benefit from the fact you own them, since it's not just the trade income, but also tarrifs from merchants present.
Europa Universalis IV
Inland trade nodes in IV. Trade nodes with access to the sea can be affected by privateers and general use of light ships to gain control of the market. With proper tactics and national ideas, it's possible to control more trade with a fleet than by directly controlling provinces within the node, especially if numerous nations compete against each other. Inland trade nodes draw entire control power from the provinces within the node, so controlling all of them provides a nation with monopoly that can't be affected by anything by outsiders. This of course provides considerable profits with almost zero investment in trade power. Special mention goes to Kraków and Xi'an trade nodes, which are at the game start almost fully controlled respectively by Poland and Ming and nothing short of extensive conquest can change that.
If an outsider nation sends its merchant anyway, they will gain a small fraction of control, but it's negligibly low and AI will rarely try doing this, having already limited resources.
The listed above Confirm Thalassocracy decision is all-powerful bonus to trade and navy. And if you happen to play as a merchant republic or a country that heavily relies on trade or plan to have a colonial empire, you are going to achieve pre-requisites anyway.
While being a very divisive subject, Colonial Nations still count for game-breaker, for a simple fact that you can establish them in crucial provinces within given trade node (thus denying other nations easy trade control) and gain the real benefit: merchants. Each CN of 10 or more provinces provides mother country +1 merchant, thus securing the merchant needed to properly manage local trade node (or send them to other side of the globe if needed).
With proper planning, this can go to another level of breaking the game. If you have a vassal and said vassal has own CNs, upon integrating the vassal, you take over his colonial possessions. This means you can have more than one CN in given region, giving you yet another merchant, while making it easier to steer trade away: rather than having to split it 50-50 with a single subject nation, you split it over two or more, so you still control majority stake in the end, often far more than 50% in total, but the rest is spread over more nations, decreasing their ability to collect trade locally due to the way how the trade node control is calculated. This technique works best with Spain and Portugal as a vassal (or the other way around), but France can use Brittany (and any other French minor that is scripted for exploration), while England has Scotland for this. Your goal in such scenario is less about integrating your vassal as soon as possible, but postpone it until about 1620s or 30s, when they have CNs of their own. This partially solves most of the issues normaly encountered with CNs, as they are smaller and weaker, thus less likely to revolt, while still providing the exact same profits and benefits.
Alternatively, deliberately staying below the 5 provinces needed for CN to form, while still taking over provinces with trade value bonus, making it an example of Bad Guys Do the Dirty Work. Let other countries waste time, money, resources and manpower on excessive colonisation and then management of CNs that will inevitably try to backstab them. All while you have direct control over the most important ports in the region and reap all the benefits of other's hard work. Just remember to fortify those provinces accordingly and laugh, siphoning all the trade generated by other nations. Meanwhile, you can use your own colonists to secure provinces in non-CN regions, thus further establishing your trade empire. This strategy is particularly viable for nations like the Netherlands or various merchant republics.
With Dharma DLC and ability to upgrade Centers of Trade to higher level, this goes further. Same with trade companies and their regions, where just controlling a single port can provide full benefits of the region.
Also from Dharma, the Mughal empire is considered to easily be the strongest formable nation in the game. Not only does it sports excellent national ideas, it also has a fully fleshed-out mission tree that will bring it many powerful bonuses, chief of which an obscenely strong permanent 10% administrative efficiency. But Wait, There's More! The Mughals also have several unique government reforms, one of which introducing the Diwan mechanism, which replaces culture acceptation: if you conquer all provinces of a culture, it is instantaneously and freely assimilated and accepted, making rebellions much less of a hassle. And as if that wasn't strong enough already, once you conquer an entire culture group, you get an unique and permanent bonus. The most likely first of which, obtained by conquering all of India, being yet another -10% to core creation cost, stacking with the -25% you get from traditions. This allows to conquer any given province and then easily afford coring, even if it's some highly-developed, globally important city, since one-third of its development won't be even accounted for during coring.
The horde government type is generally agreed to be the overall strongest in the game, owing largely to its special razing ability. After taking provices in a war and before coring them, the horde can then destroy some of their development, reducing the provinces value... in exchange for monarch points, arguably the most important resources in the game. Using this ability hordes can accumulate hundreds of monarch points in a few years and spend them on technology, developing for institutions, generals and coring costs to rapidly expand their empire. Did we mention razing rhe provices also reduces their coring cost, allowing you to take land past 100% overextension in a war without subject nations? In order to facilitate this cycle of warfare and monarch point spending, hordes also get the tribal feud and tribal conquest Casus Belli, offering them reduced aggressive expansion to avoid provoking a coalition. These perks combined allow hordes to expand faster than any other government type with very few drawbacks note Namely, a few economic maluses and overreliance on cavalry, which ideas can turn back into an advantage anyway. and make them excellent nations to attempt world conquest with.
Special mention goes to the largest horde of them all: Oirat. While it goes without saying that size matters, Oirat has a strong strategic position between three religious groups (Orthodox Christian to the far west, Sunni Islam to the south and west, and Confucian to the east) that allows it to carefully manage aggressive expansion. The latter is the most important, however, due to Oirat's unique relationship with Ming China. Ming, while powerful and obscenely rich, is also frail due to its Mandate of Heaven mechanic. If a horde nation with more than 300 development (easily achievable for Oirat) borders China and the Emperor of China fails to crush the horde, China then suffers the Unguarded Nomadic Frontier disaster, severely destabilizing the country, crashing the mandate and weakening the Chinese troops into absolute pushovers, ripe to be conquered by a healthy nation. Oirat can easily activate the disaster and hold its own against the gigantic Ming, made all the easier if the Oirat player can isolate the emperor among the armies of Ming. If Oirat specifically defeats the emperor in battle, a special event occurs note in a nod to the historical capture of Ming Yingzong by Esen Taishi during the Tumu Crisis and Oirat will immediately occupy the entirety of northern China, including Beijing. note This is a departure from the historical situation, whereby Ming simply crowned another emperor: Yingzong's younger brother (who reigned as the Jingtai Emperor). This dramatically turns the war in favor of the player and allows the medium-sized nation to take China's most valuable cities, permanently damaging the mandate and allowing the player to crush the greatest power of 1444 with ease in successive wars. This lets Oirat then develop a strong economy to power its war machine for the rest of the game.
Most of the historically significant nations from the time period have unique advantages that turn them into powerhouses when exploited by the player, especially with DLC, Some start out powerful while others benefit from extreme cases of Magikarp Power:
France, the Big Blue Blob itself, returns with a vengeance. Starting with some of the richest land in the game and a small army of vassals to throw at the enemy, France can easily throw the English out of Europe with the help of its allies, who will automatically join its defensive war against England. Afterwards, France is surrounded by weak nations like Provence, Brittany and the Irish minors that can be all be taken with no effort. Once it gets Élan, its second national idea giving a 20% Morale bonus, France can then expand into Spain, the lucrative English Channel trade node or even Italy. Even better is that France is one of the prime candidates for the Burgundian Inheritance, giving it up to 400 development if one is willing to fight a defensive war against Austria to keep it. France also gets a reduction in colonial native aggressiveness, so it can even colonize if the player decides not to expand farther into Europe. Alternatively, France can navigate its mission tree from the Emperor DLC to force a Personal Union on Spain, possibly giving it a massive colonial Empire in a single war. Making it even more versatile, France gets a 10% technology discount and close proximity to most institutions, so it can easily stay ahead of tech while belonging to the best technology group. France is a fantastic jack-of-all-trades that can perform well with any kind of playstyle and is highly accessible even to beginners.
The Ottomans are the other infamous "blobbing" nation, starting out ahead of every other nation in the game thanks to its technology group and a +5% discipline bonus, The Ottos are surrounded by a horde of weaker nations it can easily smash apart to build its power base, enhanced by its huge 20% core cost reduction to integrate land faster. Chief among those weaker nations is Byzantium, which is surrounded by the Ottomans and once annexed allow the Ottomans to get Empire-rank bonuses without needing the required 1000 development. After cleaning up its neighborhood, the Ottomans can rotate between conquering Eastern Europe, Africa and Asia to keep aggressive expansion penalties in check and expand indefinitely. The final Ottoman idea also increases force limit by 33%, allowing it to field huge armies in the endgame to compensate for the western powers getting stronger in the meantime. These factors all make the Ottomans so massive and powerful that even the AI will frequently turn the Ottomans into the #1 World Power, causing no end of grief for the player.
The Domination DLC rockets the Ottomans into new heights. First, the Ottomans now have a large mission tree covering territories it historically had, which is a lot. Second, they now get a special event chain (related to the mission tree) allowing them to absorbs their main rival Mamluks in one war. Finally, they get a special mechanic "eyalets" which are subjects made out of other nations that gives benefit similar to directly-owned provinces and allowing easy reconquest.
Castile, later Spain, becomes this with the Golden Century DLC. Castile starts as the largest power in Iberia with a 15% Morale bonus at the start of the game and a Gold mine in La Mancha, making it the strongest and richest nation in Iberia. After disinheriting its worthless 0/0/0 heir and getting through the initial conquest of Granada, Castile gets near-guaranteed access to no less than three personal unions in the opening 50 years of the game: Aragon, Naples and Portugal. Aragon is obtained through the Iberian Wedding event on most playthroughs without any effort from the player. Naples can be taken through a forced Union war if the player acts quickly enough to marry Naples and claim the throne as soon as it splits off from Aragon. Portugal can be obtained by getting a Force Union cb after getting Aragon. note It's best to avoid clicking the mission for taking Aragon until after Portugal takes the exploration idea group, as they won't adopt it otherwise and the Restoration of Union Casus Belli expires after 15 years.. Then, if one can seize the Netherlands from the Holy Roman Empire (or better yet, get most of it from the Burgundian Inheritance and fight for the last few provinces,) Spain can then claim the throne of Austria AND a non-Catholic Britain.note If Britain stays Catholic, Spain instead gets useful, but much less powerful, permanent claims on all of Britain. That means Spain can obtain up to 6 personal unions over most of the notable nations in western Europe. On top of all this (and a huge benefit for players even without DLC) Spain gets a free colonist in its national ideas and increased settler growth, letting it colonize faster than everyone else and raking in ducats sooner to fuel conquest, made all the more powerful if Spain has unions on Portugal and Britain to to do even more colonizing free of charge and granting Spain more merchants in the process. Spain can quickly snowball into an obscenely wealthy megastate with a over a dozen colonial nations and multiple Personal Unions to crush all opposition
England starts out in a difficult position, on the eve of the Surrender of Maine event (Starting the final stage of the Hundred Year's War against France) and ticking down to the War Of The Roses disaster, England has to choose between backing down and building up strength to fight France later... or, with the help of allies and mercenaries, it can defeat France and force it into a Personal Union, turning one of the best nations in the game into the player's personal attack dog. At that point, one can take the British Isles effortlessly and once France unlocks its 2nd National Idea to bolster its morale, take on Castile for complete dominance of western Europe and the New World. England starts colonization late, but to make up for this it gets the home trade node of the English Channel, a valuable "End Node" where no value can escape. If one can secure France and absorb it (and possibly Burgundy, too if England marries them first for a chance at the inheritance), then the player can have near-total control of the channel and start raking in huge sums of money as early as 1500. On top of this, British Naval ideas and Britain's island starting position make it nearly impossible for the notoriously incompetent AI Navies to land troops into the capital and valuable provinces nearby, ensuring that Britain always has a defensive edge in wars.
Later, England gained the ability to form the Angevin Empire if it wins the Hundred Years War. The insane diplomatic bonuses and the ability to gain permanent bonuses for forming Ireland, Spain and Italy as personal unions; along with the fact that you already have France under a personal union (with the aforementioned chance of Burgundy just increasing the insanity even more) and forming the Angevin Kingdom uniting the English, Welsh and Lowlander Scottish cultures into the French culture group as the Anglois means that it's essentially a later version of the Holy Roman Vassal swarm strategy of Austria discussed right below which snowballs even harder. This path goes into Difficult, but Awesome. If the Hundred Years War is conceded, England gets the older Boring, but Practical Britain mission tree which turns it into a colonial powerhouse later but potentially more powerful than Castile and Portugal.
Austria starts the game as the Emperor of the Holy Roman Empire,a decentralized network of bickering microstates that demands the player's constant attention lest they devour each other and harm your Imperial Authority, a stat which is required to pass Imperial reforms that increase the Emperor's power over time. Worse yet, the Protestant reformation threatens to start shortly into the game and rip the Empire apart (and Austria with it) in the War of the Protestant League. To even the odds and make the trouble worth it, the player gets several extra diplomatic relations as the emperor to build a large network of allies to both secure the loyalty of the electors and enforce peace on the smaller states of the Empire as they try to conquer each other. On top of this, Austria is given several "Force Union" Causus Belli: Hungary, Croatia and Milan without the Emperor DLC, and adding Bohemia and Poland (and possibly Lithuania with it) to the mix with Emperor, letting Austria have a swarm of minions early into the game. Austria is also a prime candidate to receive the Burgundian Inheritance, letting it take the Netherlands for free to either release for more imperial Authority or to add directly to the player's territory and get a cut of the valuable English Channel trade node. While the protestant reformation is still a massive threat, it can be contained by declaring war with no Causus Belli and enforcing religion in the peace deal, destroying each of the 6 centers of reformation before they can convert the empire and destroy the player's Imperial Authority. Once one gets through the reformation, it's only a matter of time before the player builds enough authority to enact the later reforms, which add a myriad of bonuses strengthening Austria's prestige, diplomatic reputation, number of allies, Imperial Authority and even end the imperial elections, all of which allow Austria to pass reforms faster over time, The accumulated powers as Emperor give Austria a superb position to make an impenetrable network of allies, vassals and Personal Union partners as Austria reaches the semi-final and most powerful reform of all: Revoke The Privelegia. This reform turns every member state of the Holy Roman Empire into Austria's vassals. A competent player will gain over 50 vassals overnight. "But what about liberty desire and diplomatic relation slots?" one might ask. Thankfully the HRE vassals are classified differently from normal vassals, and neither take up slots nor get huge amounts of liberty desire. At the same time, these vassals contribute income and force limit to Austria, allowing it to field giant armies... not that they need to do anything, as Austrias vassal swarm will devour anything you point them towards through sheer numbers. These vassals can also be fed land to save Austria coring costs and overextension, allowing Austria to take much larger chunks of territory in wars. All these factors make it perfectly possible to become an absolutely invicible superpower by 1650, likely even earlier, and despite the setup time, it makes Austria the best European candidate for world conquest by a large margin.
Brandenburg and its formable successor Prussia is one of the best military nations in the game, and can easily take down armies far larger than its own. Starting off as a tiny nation in the Holy Roman Empire, Brandenburg has to carefully manage aggressive expansion and stay in Austria's good graces. Once it can form Prussia, though, Brandenburg evolves into a powerhouse with 20% extra morale, Force Limit and Infantry Combat Ability that after stacking Military ideas is capable of taking on vastly superior numbers, giving it the advantage it needs to dismantle the Holy Roman Empire and go on to either form Germany or continue wrecking house. Prussia's only drawback is its massive 50% reduction in governing capacity, but the Leviathan expansion ameliorates this using concentrate development. Simply concentrate dev on every territory you conquer, cutting down a portion of a territory's development and adding 2/3rds of it to your capital, which costs 1% the normal amount of governing capacity to maintain. Spamming court and state houses then cuts the rest of the cost down to almost nothing and allows the player to expand well beyond Prussia's intended limits.
As weird as that sounds, Byzantium, the ultimate expression of Difficult, but Awesome in this game. The nation may start surrounded by the resident Game-Breaker Ottoman, but should it manages to survive its historical doom and reclaims its Balkan territoriesnote very doable by getting alliances early to avoid an instant death then going over the limit to build a navy that can trap the Ottomans in Anatolia, the country will receive several events and decisions that will raise its military tradition to nearly 100 before 1500. This will allow it to recruit utterly bonker generals on the levels of Skanderbeg at a time when a general's pips are all that matter in a fight. Byzantium later gained the ability to land a noble Pronoiar in it's vassals, freely inheriting them upon the new rulers death. This makes it, alongside the Ottoman Eyalets and Austria's Holy Roman swarm, the best vassal master in the game on top of the insane military bonuses.
