The Crusades were not random wars of aggression. The Crusades were the Eastern Christian response to centuries of Islamic conquest. They were primarily defensive wars. In early Christianity there were five centers of high authority: Rome, Alexandria, Constantine, Antioch, and Jerusalem. Each one had a Bishop and administered a large region, both politically and religiously. The origins were Apostolic and the cities were the core seats of Christian authority. Islam rose out of the Arabian Peninsula in the early 600s. Almost immediately after consolidating power in Arabia, the Muslim state launched rapid military expansion against its neighbors from 632 onward. Islam took 3 out of 5 of the Patriarchal Holy Sees (Alexandria, Antioch, and Jerusalem) in the 7th century. The Crusades were the Eastern counteroffensive. The Muslims had conquered the Iberian Peninsula (Spain and Portugal) and were in Tours, France by 732. It took over 700 years for the Spaniards to expel the invaders. The Reconquista was a parallel Western front. The Ottomans laid seige to central Europe for hundreds of years. They took the Balkans and Constantinople fell in 1453. Vlad Tepes of Wallachia (reimagined in modern times as the horror character Dracula) fought them off heroically for 6 years and drove them out of his principality. They laid seige on Vienna in 1529 and were expelled from Vienna 160 years later. Only Rome remained from the original Holy Sees after 1453 when Constantinople fell. Things you don't learn in school these days.
