Final answer:
The Fourth Crusade was an expedition initially intended to recapture the Holy Land but ended up sacking Constantinople. It is called "the crazy one" due to its diversion from the original objective and attack on a Christian city.
Step-by-step explanation:
The Fourth Crusade was a military expedition launched by European Christians in 1202 with the goal of recapturing the Holy Land from Muslim rule. However, it took an unexpected turn and ended up resulting in the sack of Constantinople, the capital of the Byzantine Empire. John Green refers to it as "the crazy one" because it deviated from its original objective and turned into a chaotic and destructive campaign against a Christian city.
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