Before Islam began, there were five main centers of Christianity: Rome, Constantinople, Alexandria, Antioch, and Jerusalem.
As Islamic warriors conquered the Middle East and North Africa, three of those centers were lost. Most of what was once "the Christian world" became what is now "the Muslim world."
Two of the centers of Christianity remained, and one of them was the new Islamic target: Constantinople (now known as Istanbul). Constantinople was a huge walled city, and it was being threatened by the now powerful Islamic empire.
Constantinople's emperor pleaded to Rome for help.
Rome needed to somehow unite the quarrelling, constantly competing countries of Europe in order to defend Constantinople. But how? The only thing the European countries had in common was Christianity, so that was what the Pope used for his call to action. "Christianity is being threatened," he said. Jerusalem is in the hands of Muslims. Their "Christian brothers" who made pilgrimages to Jerusalem were being slaughtered. "Constantinople has cried out to us for help. We must save Christianity."
The appeal worked. People from many European countries put aside their differences and joined in the defense of their culture.
Several Crusades were organized over the next two hundred years. They didn't save Constantinople, but they did slow down the conquering forces so that Rome and Europe mostly survived. I say "mostly" because Italy and Spain were invaded by Muslim armies and Spain was conquered by Islamic forces for 800 years.
Spain finally re-conquered their own country in 1492, the same year Christopher Columbus "discovered" America while he was trying to find a new trade route to the East (the old trade route had been cut off by the Muslims who had conquered the Middle East and didn't want any infidels travelling through their territory).
The Crusades began as a response to a call for help because Muslim armies had conquered most of what had formerly been "the Christian world" and were threatening what was left of it with annihilation.
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