What was the main cause of the turmoil that led up to the crusades?

While the Crusades ultimately resulted in defeat for Europeans and a Muslim victory, many argue that they successfully extended the reach of Christianity and Western civilization. The Roman Catholic Church experienced an increase in wealth, and the power of the Pope was elevated after the Crusades ended.
Trade and transportation also improved throughout Europe as a result of the Crusades. The wars created a constant demand for supplies and transportation, which resulted in ship-building and the manufacturing of various supplies.

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