Kruistocht in Spijkerbroek, known in English as Crusade in Jeans, is a Dutch children's book written by Thea Beckman featuring the child crusade from 1212.
Rudolf "Dolf" Wega is a volunteer for an experiment with a time machine, which sends him to the thirteenth century. However, he misses his opportunity to return, and gets stranded. This leads him to go along with a child crusade, led by shepherd's boy Nicolaas, whom everyone believes will lead to children to Jerusalem by splitting the sea at Genua like Mozes. Nicolaas is supported by the monks Dom Anselmus and Dom Johannes.
Using twentieth century knowledge, Dolf tries to help out, causing him to make both friends and enemies.
An English language film version with the same title was made in 2006.
Kruistocht in Spijkerbroek contains the following tropes:
- Been There, Shaped History: Turns out that Leonardo, who became Dolf's friend early on, is famous mathematician Leonardo Fibonacci da Pisa, who was introduced to Arabic numbers (so 1,2,3... et cetera) by Dolf. Therefore, Dolf made a major contribution to modern mathematics. Although actually this is a anachronism, because Fibonacci made publications about this before the events of the book.
- Changed My Jumper: Guess which piece of clothing he wears that is definitely out of place in 1212.
- Get Back to the Future: Part of the premise of the book.
- Giving Radio to the Romans: Dolf constantly uses his knowledge from the future to improve their circumstances, which sometimes even saves lives. Most of the time he doesn't really build something futuristic, except for that time he manufactures gunpowder for trickery.
- Heel–Face Turn: Johannes, who begins to feel contempt with their plan to sell the children as slaves in Africa. He starts bawling his eyes out as his conscience increasingly gnaws at him, causing him to confess his sin to Dolf and his allies.
- Messianic Archetype: Subverted by Nicolaas. He was conned into believing he's on a Mission from God by the two monks accompanying him to send the children of the HRE on a crusade to liberate the Holy Land but actually to sell them into slavery. He's not particularly wise or noble, merely very pious and Holier Than Thou. One of the monks later points out that their "burning firebush" trick completely failed with the first two boys they tried it on, and it's only because Nicolaas was so full of himself already that he readily accepted that he must be some sort of savior.
- The Plague: The crusade is beset by a plague that claims the lives of quite a lot of children. Dolf, being from the future, understands germ theory and implements quarantine and hygiene measures to stop the disease. He explains it to the medieval people he's surrounded by in religious terms, describing bacteria as tiny demons sent to sicken the lord's chosen.
