Probably the most successful crusade since the First, although largely without any military incident in the Holy Land. Frederick II of Germany, who had failed to keep his promise to join the Fifth Crusade, married Yolanda, daughter of John of Brienne, king of Jerusalem, and claimed the title for himself. His relations with the papacy had been soured by his failure to help the Fifth Crusade, and he was under severe pressure to mount a new crusade. However, a first attempt to reach the Holy Land was foiled by an outbreak of fever amongst the crusaders, and Frederick was forced to turn back. Pope Gregory IX did not see this as a valid excuse, and when Frederick again set off in 1228, Gregory excommunicated him, and preached a crusade against Frederick, occupying his lands around Naples. This also affected Frederick when he arrived in the Holy Land, and very few of the crusaders already there obeyed his orders. Once there he entered negotiations with Malik al-Kamil, Sultan of Egypt, and ruler of Jerusalem, and gained Jerusalem, Nazareth and Bethlehem, along with a corridor giving access to the sea. He entered Jerusalem on 18 February 1229, where he was crowned King of Jerusalem, before returning to Italy, where in May he regained control of Naples, before finally making peace with the Pope in August. While Christian control of Jerusalem only lasted fifteen years, Frederick's achievement was impressive, gaining what previous crusades had failed to do at so much cost through clever diplomacy.