The Fifth, 1221
In order to finance the Fifth Crusade, a tax was imposed. The plan for this crusade was to enter Jerusalem by way of the sea and then go on to Egypt because Frederick II was headed there. They planned to meet there, gather forces and go back to fight the Muslims, who had just let them pass through Jerusalem. Frederick never showed up, but the Muslims knew that the Crusaders were camping on a flood plain and so opened the floodgate and the crusaders were washed away.
The Sixth, 1228
Well, it turns out that Frederick II had become friends with the Muslims, and decided to team up with them instead of the Christians. The leader of the Muslims was at this time Al-Kamul. He and Frederick decided to share Jerusalem and let the Christians visit as long as the Muslims were also allowed to worship there. The Pope was not happy about Frederick's behavior in this matter and so excommunicated him and didn't allow Christians to pilgrimage to Jerusalem.
The Seventh, 1254
Next the Turkish Muslims attack the remaining Christians in Jerusalem. Louis IX of France decided to take matters in his own hands and led a crusade. Despite his good intentions, his army had to retreat to regroup. It didn't help that his army suffered from dysentery.
The Eighth, 1291
King Louis of France decided that he needed to team up with the Mongols against the Muslims. The Muslims, however, had meanwhile began abducting Russian slaves and training these youths to be ruthless warriors called Mamelukes. The Muslims and the Mamelukes devastated the Christians, therefore taking control of Jerusalem.