We use the term "crusade" these days to mean a moral struggle, people rallying themselves to fight for the right (for example, World War II is sometimes described as a "crusade"). But look at the word; at its root it's something to do with a cross. Its core meaning is Christians fighting for Christ, which in the Middle Ages usually meant killing Muslims (it's too late now to tell them it was a very bad idea, but we're still dealing with the aftermath).
(Interestingly, what we call a crusade was in the twelfth and thirteenth centuries just called going to Jerusalem.)
The First Crusade was successful beyond anyone's expectations, leading in 1100 to the establishment of the Christian Kingdom of Jerusalem. As I've discussed elsewhere, winning essentially ended after that. One of the biggest disasters (though it's hard to choose) was the Fourth Crusade of 1202.
This was after the 1187 fall of Christian Jerusalem and the failure of the 1189 Third Crusade to get it back. But by 1202 hopes were high for victory. A big army was assembled. Planning to go by sea (rather than overland, since after all Frederick Barbarossa had died trying to go overland in 1189), the main army arrived in Venice, which had been busy building ships for them.
Here the Venetians came up with a startlingly large sum for their ships. (Part of the problem was that the Venetians had been told to build ships for about three times as many Crusaders as actually showed up--and had done so, and didn't want to get stiffed for all that inventory.) The Crusaders didn't have nearly enough money, and it was going to take more than a couple of bake sales to raise it. So the Venetians suggested a compromise: they would take much less if the Crusaders would first sack Zara, a trading rival to Venice, located in Croatia on the opposite coast of the Adriatic.
Zara was Christian (indeed Catholic), but the Crusaders needed those ships, so they took the deal. The pope, understandably, was distraught when he learned about it and told the Crusaders to come home and be excommunicated. Instead, trying to make up for this inauspicious beginning, the Crusaders pushed on, sailing to Constantinople.
The plan was to hook up with the Byzantines and together attack the Muslims in the Holy Land. Popes had been hoping for over a century that such joint efforts might reconcile Latin Christendom and Greek Orthodoxy (though you'd have thought by now they would have figured out that it wasn't working). Meanwhile, political infighting was going on in Constantinople, and the Crusaders were met in early 1203 by a claimant to the imperial throne who asked for their help in getting his "rightful" rule.
They threw themselves into it with enthusiasm. Although they got the claimant crowned, one thing led to another, and in 1204 the Crusaders ended up sacking Constantinople (the claimant had been murdered in all the excitement). Then they looked around and realized they'd just slaughtered a whole lot more Christians. Whoops.
Very delicately worded letters were sent to the pope, saying how delighted he'd be to learn that Constantinople was now fully committed to Latin Christendom, no more schism here! The pope even wrote back with congratulations before figuring out what had really happened.
The leaders of the Crusade headed west to "explain" more fully what had happened and to take home some of the great relics they'd obtained. Meanwhile, since the westerners after all now controlled the city, they declared a Latin Empire of Constantinople. Multiple French lords competed for who got to be emperor. This Latin empire lasted only two generations, before the Byzantines took it back in 1261. The long-term effect was just to weaken Byzantium, although it managed to hold on against the Turks until 1453. But the Fourth Crusade never got anywhere near Jerusalem.