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What if in 1204 as the crusaders besiege Constantinople they cant take the city. Say a big storm like the one which affected the Arab fleet earlier occurs. Say that on land a plague and disease start to destroy the crusaders and the Bulgarians reach Constantinople in time lifting the siege of the city. While Constantinople impenetrable walls hold out.
What happens if the siege fails and the crusaders suffer a crushing defeat crippling their army.
With the siege failed how does world history change?