Pope Honorius III began preaching this crusade very shortly after rising to the papacy. After the failure of the Third and Fourth Crusades (enacted by his predecessor, Pope Innocent III), Pope Honorius II was eager to retake the Holy Land. Though the monarchs of Europe contributed little to this crusade, the pope found sufficient support among the French and German barons. The Frisians offered a small fleet of ships, though they lent few soldiers to the cause.
Egypt was decided to be the crusaders' original destination, for a couple of reasons: Even if they managed to take Jerusalem directly, it would fall quickly to the Muslim forces within Egypt. However, if the crusaders could seize Egypt, they could not only attack the exposed flank of Jerusalem, they would do so without having to worry about their back being vulnerable to attack. Additionally, the wealth of Egypt was probably quite attractive to the greedy leaders of the crusade. On May 24, 1218 the crusaders set sail for Egypt.
The campaign for the Holy Land began with an assault of Damietta, a town that guarded Cairo by stopping unwanted ships from traveling along the Nile. Damietta itself was guarded by a small tower in the waterway just outside of the town. It took the crusaders considerable effort and nearly a month to take this tower, which fell in mid-August. By the time they had taken the tower, the soldiers were too few to siege Damietta, and so had no choice but to wait for reinforcements, which arrived in September.
With the arrival of new soldiers, a siege was begun against Damietta. But, with the Egyptians devoting their resources primarily to defending the town, the siege quickly became a stalemate. Then, in early February, a Kurdish chieftain named Imad-ad-Din, who was one of the Egyptian Sultan strongest supporters, began to plot against Egypt. At news of this, the Sultan fled for Cairo, and in the ensuing chaos, the Sultan's armies fell back, leaving Damietta undefended.
The Egyptian Sultan, greatly worried about potentially losing Damietta, offered to give Jerusalem to the crusaders in return for a thirty-year truce. The crusaders, confident that by capturing Damietta they could take the whole of Egypt in addition to Jerusalem, decided to refuse the offer. This would turn out to be the greatest mistake of their campaign.
In March, Egypt received reinforcements, and managed to reassert their defensive position in Damietta, despite multiple attacks by the crusaders. Over the next several months, the crusaders failed in numerous assaults on Damietta and other nearby Egyptian forces, sustaining heavy losses. With morale low and military leaders growing impatient, the Christian army made a valiant (though ill-advised) strike at the Sultan's camp, but were quickly routed, losing thousands of soldiers.
The Sultan once again offered the city of Jerusalem to the crusaders, but the offer was rejected. They were relying on a large army of reinforcements that had been promised by Emperor Frederick II of Hohenstaufen, one of the greatest monarchs of the day. In early November, shortly after they refused the offer, the crusaders finally managed to take Damietta by way of an unattended watchtower. Unsure of what to do, the Egyptians pulled back and waited to see the Christian army's next move.
The Christians, however, with no true leader, opted to wait for Frederick II and his army before they committed to any course of action. The Emperor decided to stay in Europe, but most of his army arrived in June of 1221. With their newly increased numbers, the crusaders decided to make an all-out assault on the Egyptian forces. The Egyptians, seeing that they could not defeat the Christians in a fair combat, expediently retreated to the far banks of the Bahr as-Saghir (a river that connects to the Nile).
Fearlessly advancing on the Egyptian position, the crusaders marched to the point where the Nile and the Bahr as-Saghir, and right into a trap. As summer turned to fall and water levels rose, a channel filled in behind the Christian army, cutting off their easy retreat. In a desperate situation, the crusaders decided to use the cover of night to retreat over the small path of land that was left to them.
Unfortunately for the Christians, the Sultan foresaw their retreat and had flooded the land. And so the Egyptians descended on the crusaders, who were hindered by darkness and mud. The crusaders were easily routed. Having their leaders at his mercy, the Sultan demanded that the Christians leave Egypt and agree to and eight-year truce. With no other options available to them, the Christians agreed to these terms.
The fifth crusade had failed.
See also General Wesc's The Crusades node.
Bibliography:
http://www.fordham.edu/halsall/sbook1k.html
http://latter-rain.com/ltrain/crufiv.htm
http://crusades.boisestate.edu/5th/
http://people.westminstercollege.edu/faculty/mmarkowski/sscle/ssclechr.html