The Crusades were a series of religiously motivated wars occurring in the 11th through the 13th centuries, fought primarily throughout the Middle East. The Crusades began after Muslim nations captured Jerusalem, a city holy to Islam, Judaism and Christianity. The Catholic Church launched the First Crusade in 1095 to take back the city, and nearly constant warfare followed for the next 200 years.
In total, there were eight major invasions against Muslim nations in the Middle East, and one ill-fated "Children's Crusade" that resulted in the deaths of thousands of young and poor people. During the course of the wars, many cities in the Middle East were captured and recaptured, and national lines were drawn and redrawn as the result of conquests. During this time there were also minor Crusades against Muslims and pagans in Europe, most notably on the Iberian peninsula.
While the ostensible motivation of the Crusades was religious in nature, many of those who participated sought only material gain and glory. The regular pillaging of cities in the region led to some mixing of cultures, exposing Europe to aspects of Islamic culture and advancement and vice versa. By the end of this series of wars, the Catholic church had grown in power and opposition to the Eastern Orthodox church, and the Mamluk Dynasty in Egypt finally became powerful enough to prevent further invasions.