The Crusades (1095-1291 CE) were a series of military campaigns by Christians primarily against the Muslims holding Jerusalem. The purpose was to regain control of Jerusalem—called the "City of God" by Christians—the city where Jesus had taught and was crucified. Muslims were equally determined to hold the city, where the Islamic prophet Muhammad had taught. This led to some of the most savage combat in human history, terrible atrocities by both sides, and the loss of millions of lives. The estimated population of Europe in 1200 was 60 million; however, since the average life expectancy was about 35 at that time, the deaths of 3-5 million adults could have meant the loss of almost a fourth of the adult population of Europe. Given that the Black Plague would develop in Europe in 1347, wiping out a third of the entire population in five years, it was a perilous time.

Note: The Children's Crusade consisted of two groups totaling 50,000 children. Neither group reached the Holy Land, so none died in battle. However, many died from starvation, exhaustion, or exposure, and when their boats never reached shore, it was presumed many were drowned or captured and sold into slavery. Almost none of the 50,000 returned home.