February 5th, 2015 
About 450 years before the first crusade began, a new religion was born in the Middle East. At the time, the Middle East was mostly made up of Christians and Jews.
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.