The chief cause of the first quarrel between Frederick and the pope was Frederick's refusal to keep his vow to go on a crusade. In 1215, on the day he was crowned king, he vowed to make a crusade, and again in 1220, when crowned emperor, he renewed the vow. For various reasons he several times put off going. Each time the pope was deeply disappointed, but eventually accepted the emperor's excuses. Again in 1225 he renewed his vow and set the time of his departure in August, 1227. But the pope had lost confidence in Frederick, as well as his patience. He stipulated that if the emperor did not keep his word, he should be excommunicated. Frederick sailed Aug. 8, 1227, but returned to land two days later. On this account Gregory IX excommunicated him, Sept. 29, 1227. Frederick published an apology for his conduct and called a crusade to take place the following May. Without seeking to have the excommunication removed, he sailed in June, 1228. For this the pope renewed the excommunication. While Frederick was absent in Palestine, his imperial vicar in Italy came into actual conflict with the papal officials about matters of government. When Frederick returned from Palestine in 1230, the pope was hardly prepared to carry on the war. So through the intercession of various princes the peace of San Germano was brought about. The preliminary agreement is found in no. 140. The papal stipulations are contained in no. 141. In order to convince the pope of his good intentions and to renew friendly relations with him, Frederick made him a visit soon after the peace was established. The pope wrote a friend an account of this visit, which is found in no. 142.