Frederick II (left) meets Al-Kamil (right).
The Sixth Crusade began in 1228, seven years after the bloody failure of the Fifth Crusade. Federico II of the Hohenzollern House of Prussia, was the Holy Roman Emperor and had terrible relations with Pope Gregory IX by that jealousy between the altar and the throne.
St. Francis meets Al-Kamil
When Frederick, had to be returned from his trip to Syria by a plague, Pope accused him of cowardice, insulted him by calling him "Antichrist" and excommunicated him. The following year, in the summer of 1229, Frederick II began his crusade again, despite being excommunicated.
Francis and Al-Kamil.
Frederick came to Jerusalem with peaceful attitude and without fighting, achieved a peaceful negotiation with Sultan Al-Kamil in which Christian pilgrims could go to holy places (places where Jesus lived) undisturbed. A diplomatic success did twist the Pope of envy.
Al-Kamil Muhammad al-Malik was the Sultan of Egypt and Syria, Saladin's nephew and son of Al-Adil who died during the Fifth Crusade. The Sultan Al-Kamil had offered several peace alternatives to Christians, but the pope wanted the Holy War. It seems that Al-Kamil had also negotiated with Francis of Assisi, who sought more peace and coexistence that the conversion of the Sultan.
Statue of Frederick II at the Palazzo Reale, Naples, Italy.
Remember that Frederick II was married to Isabella II of Jerusalem, daughter of Corregidor Jerusalem (Mayor appointed by the King), Jean de Brienne. Therefore, Frederick had inherited interests to the throne of Jerusalem. So it was not just diplomatic skills to achieve the go-ahead to the pilgrims but also because it had political interests involved.