The Fourth Crusade (1201-1204) ran aground when it was seduced into a web of Byzantine politics, which the Westerners never fully understood. They had made a detour to Constantinople to support an imperial claimant who promised great rewards and support for the Holy Land. Yet once he was on the throne of the Caesars, their benefactor found that he could not pay what he had promised. Thus betrayed by their Greek friends, in 1204 the Crusaders attacked, captured, and brutally sacked Constantinople, the greatest Christian city in the world. Pope Innocent III, who had previously excommunicated the entire Crusade, strongly denounced the Crusaders. But there was little else he could do. The tragic events of 1204 closed an iron door between Roman Catholic and Greek Orthodox, a door that even today Pope John Paul II has been unable to reopen. It is a terrible irony that the Crusades, which were a direct result of the Catholic desire to rescue the Orthodox people, drove the two further—and perhaps irrevocably—apart. That's putting the results mildly. The 4th Crusade did more to help the Muslims than just about any other military blunder in history. Up until then, the Byzantine Empire -- and Constantinople in particular -- had held out strongly against Muslim attempts to spread into the Balkans. After it was sacked, a Norman ruler was installed for a little more than half a century. The Byzantines eventually got their country back, but the empire and Constantinople were so weakened that it was only a matter of time -- about 200 years as it turned out -- before the Muslims broke through.
Almost all the religious strife in the Balkans these days can be traced back to the 4th crusades attack and sacking of Constantinople -- now called Istanbul.