I'm wracked by procrastination, so I'll just wittle away the minutes on this. Steven Runciman was a famous medieval historian, he wrote a landmark series called 'History of the Crusades' that's basically on every library shelf. His book Byzantine Civilization is still one of the best on the Byzantine Empire, despite dating from the 1930s.
Anyway, as a pro-Hellenist, he condemns the Crusades, in particular the Fourth Crusade (although the earlier ones came in for their fair share of criticism too) as having irreversibly weakened the real bastion of Christendom. He was the one who coined the famous phrase: "the Holy War itself was nothing more than a long act of intolerance in the name of God, which is the sin against the Holy Ghost."
I'm not going into why the Fourth Crusade happened, it's a long and complicated event that may have happened by accident. The Westerners came and broke the empire in 1203-4 and then couldn't put it back together again. Latin self-interest just fed into the political chaos.
In History of the Crusades (volume 3, 477) Runciman writes that the Byzantium Empire was so weakened that it could 'no longer guard Christendom against the Turk', which lead to 'persecution and slavery' for the 'innocent Christians of the Balkans'. Basically the Ottomans conquered large swaths of the Balkans, and Runciman supposed that the Byzantines might have kept the Muslims from Europe a while longer yet, if only their Christian brothers hadn't dealt them such a devastating blow. He may have been right.
The unified bulwark, which had withstood three Caliphates and repelled Muslim assaults and raids for centuries, fragmented into separatist Greek states. Even when Constantinople was retaken by the Greeks in 1261, it remained a shadow of its former self. The Byzantines from then on appealed to Western help all the time, using union of the two Churches as the drawing card. A sad, pathetic state of affairs.