The Crusades were a disaster for Christianity because they hastened the collapse of the Byzantine Empire and made the schism between East and West permenant.
What pray tell was the justification of the Fourth Crusade and the sack of Constantinople in 1204?
Or the burning of Greek monks at the stake?
The Byzantine Emperor petitioned the Pope for a united front against Islam, but instead the Franks turned on the Byzantines.
Had Christendom been united and the West helped rebuild the Byzantine Empire, it might not have fallen in 1453.
Byzantium signed the Union of Florence hoping for a new Crusade, but all that happened was the West turned its back on its Christian brethren in the East.
Frankly I blame Western Civilization on my closer relatives ~ "The Family" and "The Bourbonaise" ~ otherwise it's anybody's guess.
Fur shur my Sa'ami ancestors had nothing to do with any of it!
Byzantine politics mixed with Venetian politics undermined this -- the sack of Constantinople 808 years ago was deplorable, yet was the result of a lot of mistrust, including the massacre of the Latins a few years earlier.
This was due to Venice -- Venice was part of the Byzantine Empire for a long time, so in many ways it was a war between brethren and the Venetians, smart cookies that they were, used the Franks as their weapon. Remember that even the sack was the Venetians diverting the attention of the Crusaders.
We add in the fact that the Byzantines looked down on these barbarians from the West (quite rightly, we Westerners WERE barbarians compared to the Easterners and we sent our warriors, not our civilized folks).
the disunion was due to politics -- not Frankish, dumb bricks that they were.
the problem with 1453 was that ahem, the Empire's attention was diverted to problems in the north (can't mention what exactly in a caucus thread!)