The Crusades were a series of holy wars in which Christians fought Muslims, attempting to regain land from the Muslims, including the Holy Land, and convert new followers to Christianity.
The First Crusade began in 1096, under the dominion of Pope Urban II. Bands of skilled noble warriors (remember the trained knights?) headed east toward the Holy Land. Most of the warriors came from France and before reaching the Holy Land they successfully conquered Anatolia. When they reached Jerusalem in 1099 there were approximately 10,000 soldiers! The city of Jerusalem was finally conquered after a deadly five-week siege and a horrible massacre of its inhabitants.
At first, the Muslims were ineffective in fighting against the armored knights. But by the 1100s, they started to fight back. In 1144, Edessa became the first Christian state that the Seljuk Turks, a group of Muslims, reconquered. When Edessa was reconquered, Christians called for a Second Crusade to get it back. At the call of Pope Eugene III, both France and Germany sent troops (separately) for a Second Crusade. Unfortunately, both sets of troops were defeated miserably.
In 1187, a sultan named Saladin invaded the city of Jerusalem and retook it from the Christians. Unlike when the Christians in the First Crusade conquered Jerusalem, Saldin forbade the killing of civilians and for a time there was even trade between Christians and Muslims. Eventually, however, Christians called for a new Crusade to conquer Jerusalem back from the Muslims again. Three European countries -- Germany, England, and France -- sent soldiers to fight in the Third Crusade. Ultimately, the Christians regained part of the Holy Land but lost their fight to conquer Jerusalem. They did, however, negotiate a deal allowing Christian pilgrims access to Jerusalem.
In 1212 a young German named Nicholas of Cologne said he was inspired by God to lead a Children’s Crusade to the Holy Land. The goal was for children to convert Muslims to Christianity through peaceful means. Thousands of young people joined Nicholas’s cause, though many turned back when the Pope urged them to go home. Unfortunately for those who journeyed to the Holy Land, most were eventually sold into slavery.