Much is said and written about the Crusades - and some of it is even true! We may note three phases: from 637 A.D. (Islamic armies conquer Jerusalem) until 1095 A.D., there is a period of unchallenged Muslim military expansion, included the invasion and sacking of Spanish cities, and southwestern France. The second period would start in 1098 (the beginning of the first Crusade) and end in 1250 (the end of significant Crusades); this would be the phase of counter-attack by Europe in response to the first phase. The third and final phase would begin in 1250 (the last serious attempt to settle or calm the source of attacks on Europe; after this, the aggression toward Europe, displayed prior to the beginning of the first Crusade in 1098, reappears.
But what is the cultural legacy of the Crusades? In a book entitled The Humanities in the Western Tradition, written jointly by The City University of New York and The University of Akron, the authors note that an important architectural example, "the cathedral Santiago de Compostela ... was destroyed by Muslims in 997." Much valuable artwork was lost in the Islamic attacks on Europe, attacks to which the Crusades were a response.
The same book notes that the scientific, philosophical, mathematical, and political developments of Europe would have been lost "if the Arabs had been able to break through Byzantine defenses and advance into eastern Europe." Imagine - no calculus, no modern physics, and no theory of government resting upon equal participation and freedom of expression!
If Europe had not been defended, history would indeed be very different!