Kruistocht in Spijkerbroek, known in English as Crusade in Jeans, is a Dutch youth 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 shepherds boy Nicolaas, of whom everyone believes will lead to children to Jeruzalem by splitting the sea in 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.
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.
- HeelFace Turn: Johannes, who begins to feel contempt with their plan to sell the children as slaves in Africa.