There are three distinct ways to rain down men into an area - traditional static line drops, as well as the specialized HALO and HAHO taught to special operations units. Each has their distinct advantages and disadvantages.
Static line drops are used by mass paratrooper operations like those from World War Two and are still used by large scale units like the 82nd Airborne. The parachute is opened immediately, as the parachute line attaches inside the aircraft and pulls the chute from its deployment bag (D-bag). These parachutes are intentionally limited in their ability to maneuver so as to prevent a scattered drop. Among other problems in modern times such operations can only be carried out against targets without even shoulder launched anti-aircraft missiles as it requires aircraft fly relatively low and slow. A variation on static line systems are also how cargo is generally dropped, with loads including light artillery, jeeps, and supplies. Tanks have been tried but are generally too light to be effective. Another odd use case is BASE jumpers, who need their parachutes to open instantly.
HALO aka High Altitude Low Opening is a technique in which the paratrooper freefalls for an extended time and opens the chute when he hits terminal velocity. This is the standard way to skydive as depicted in movies and TV as well as what the typical civilian skydiver does. Most parachute demonstration teams such as the US Army Golden Knights also use this technique for demonstrations to civilians. However, HALO jumps have the disadvantage of inaccuracy - it is much more difficult to steer while freefalling - and noise due to terminal velocity being quite high. They are therefore not actually ideal for inserting covertly. Therefore a large HALO jump involving over a hundred men may require a very large drop zone, and is extremely dangerous to pull off at night. You might see a lot of instances of Parachute in a Tree if so used. This kind of jump is therefore for the most part Awesome, but Impractical in an actual military sense. There is a useful variation originated by US Navy SEALs as a way to drop with their boats into the ocean, also used by other special operations units over land as a way to drop with vehicles. By dropping with vehicles often over the horizon they then travel on the surface and are less easily detected.
HAHO or High Altitude High Opening, also known as Military Free-fall is a more recent technique, that is the Boring, but Practical brother of the HALO jump. This method involves jumping at an altitude of more than ten thousand feet and deploying your chute immediately. You then slowly glide down to your target. The ram-air chute also allows you to steer with higher accuracy due to the high altitude, compared to HALO jumps where you fall fast but fall where the wind takes you. Also, since you never hit terminal velocity, and slowly glide down to your drop zone, your insertion is a lot quieter compared to a HALO jump. And your radar profile is also significantly reduced, making the HAHO the preferred method for a covert precise insertion. It does have disadvantages though. The high altitude and long glide time means you will need oxygen and cold weather gear - it gets bone chilling cold at high altitudes. You will also experience air pressure changes as you glide down. Therefore, you are at a higher risk of hypoxia due to oxygen tank malfunctions, and decompression sickness from the pressure differential. Therefore, a high level of training is a must for this. Typically only spec-ops personnel receive such training. An interesting variation on this technique is also used for cargo using the same style of ram-air parachute and a GPS guidance system.
Obsolescence of Airborne operations
By the end of World War II, it was questioned just how useful airborne operations really were in most cases. German paratroopers convinced everyone else it was a good idea with their success at the Battle of Crete, but they took such heavy casualties that they swore they would never do it again. The landings at Normandy were probably worth it as a means to disrupt German responses to the landings, but those in Market Garden as shown in A Bridge Too Far were not. The bottom line with airborne units is that they are far too easily left unsupported behind enemy lines without enough heavy weapons or ammunition to win. They are best used in relatively small scale operations in which they will be quickly relieved. For most of the war the various airborne units were best used as semi-elite light infantry that often rode around in trucks(US Army) or just walked(German Army).In 1954, just after the The Korean War, Lieutenant General James Gavin, the former commanding general of the 82nd Airborne in World War II, wrote a book called “Cavalry! And I Don’t Mean Horses” in which he outlined the idea of using helicopters to create a new airmobile cavalry. His idea took the idea of an airborne force being a rapid deployment force but expounded on the concept that while paratroopers could be rapidly deployed, they couldn’t then be rapidly redeployed. Paratroopers have to seize an “air-head” staging area, then seize an airfield or airstrip on which airplanes can land. Only then could paratroopers be airlifted to their next drop. This, according to General Gavin meant that airborne troops could only be used for very specific offensive operations. With helicopters, Gavin argued, you got a lot more flexibility. Since they are VTOL aircraft, helicopters were as useful in air insertions if troops as parachute drops. But then, helicopters, due to their hovering ability could insert troops in more places than a parachute drop could. Furthermore, those helicopters could easily be used to then redeploy that landed force as needed, as they didn’t require an airstrip like fixed wing craft did. Heavier helicopters like the CH-47 Chinook also absorb much of the cargo supply role, with the CH-53 Super Stallion even being capable of airlifting Marine LAVs. Even covert HAHO insertions are becoming increasingly rare, with infiltration helicopters that can do aerial refueling like the MH-53 Pave Low or MH-60 Blackhawk being able to fulfill not only the covert insertion role but covert extractions too - as demonstrated in the raid that killed Osama bin Laden.
Gavin initially faced opposition from Army traditionalists, who saw helicopters only as medevac busses, and not as offensive weapons. The marines however, saw its advantages in amphibious and expeditionary warfare, and adopted helicopters early. Eventually, the Army came around, and even converted the 101st Airborne into the 101st Airmobile just when The Vietnam War broke out. That conflict drove the final nail in the coffin for large scale air drops by proving just how useful heliborne troops can be, particularly in an asymmetric conflict. This was proven by the 82nd Airborne in Iraq mostly riding around in Humvees to their destinations instead of parachute drops. Fairly large airborne operations were done in Panama (1989) and Northern Iraq (2003), but in both cases it was of questionable effectiveness and arguably a publicity stunt by airborne commanders asserting their relevance. In Panama the airfield had already been seized by Army Rangers and US Air Force combat controllers, while in Iraq most of the unit just landed on the otherwise empty airfield directly. Those that landed were actually ready to fight sooner because they didn't have to reorganize. Most airborne units are actually just semi-elite light infantry in a similar category to Marines that are best treated as airmoible. The aircraft are also better used for pure logistics rather than being diverted and sometimes lost in airborne operations.
