The Byzantine Empire (Constantinople, modern Istanbul), had been fighting off Muslims for years. They lost Egypt, Syria, etc to the Muslim hordes going across Africa and into the Middle East. They asked the Pope for help from western Europe especially after the French defeated the Muslims at Tours, France trying to take Europe from the west. At least they drove them out of Spain and Portugal. The crusaders sacked Constantinople because they saw all the gold and the wealth of the city and wanted it for themselves instead of taking back the old Byzantine empire from the Muslims. They also spread into India and into central Asia and down the coast of east Africa and all the way to Indonesia before finally running out of steam. Now they are back trying to expand again and are deceiving many people, especially the poor.