The majority of the Crusades were fought over a period of about 200 years. Many historians view these conflicts as initially defensive actions to protect Christendom, primarily undertaken in response to centuries of Muslim military campaigns of expansion.
In 1095, Pope Urban II called the first Crusade to defend the Christian West after Muslims attacked Constantinople. The Christians fought back and recaptured Jerusalem. It is estimated that approximately 1.7 million people died in various battles between Christians and Muslims over the course of a couple centuries. By comparison, the black plague killed between 75-200 million people in just seven years during the time of these Crusades. This offers some perspective, though it doesn't justify any injustice and evil done in the name of Jesus.