Why did the Crusades occur?

Numerous social, religious and economic factors contributed to the launch of the Crusades, but primary among these was Europe's growing capacity for conquest. Having recently recovered from the Dark Ages, the nations of Europe were able to launch large-scale military action for the first time in centuries.

European nations had endured several massive invasions from outside forces. Casualties from these invasions kept the population of landowning nobility down. However, after the invasions were repelled, there were more landowners than there was land to support them. A need to expand their territory and acquire more land caused many nobles to support the Crusades.