King Philip of Swabia, who intended to use the Crusade to install his exiled brother-in-law, Alexios IV Angelos, as Emperor. This required the overthrow of Alexios III Angelos, the uncle of AlexiosIV. Alexios IV offered the crusade 10,000 troops, 200,000 marks and the reunion of the Greek Church with Rome if they toppled his uncle Emperor Alexios III. When the crusade entered Constantinople, AlexiosIII fled and was replaced by his nephew. The Greek resistance prompted AlexiosIV to seek continued support from the crusade until he could fulfil his commitments. This ended with his murder in a violent anti-Latin revolt. The crusaders were without seaworthy ships, supplies or food. Their only escape route was through the city, taking by force what Alexios had promised and the new anti-westerner Byzantine rulerAlexios V Doukasdenied them. The Sack of Constantinople involved three days of pillaging churches and killing much of the Greek Orthodox Christian populace. This sack was not unusual considering the violent military standards of the time, but contemporaries such as Innocent III and Ali ibn al-Athir saw it as an atrocity against centuries of classical and Christian civilisation. Fifth Crusade The Fifth Crusade (1217–1221) was a campaign by Western Europeans to reacquire Jerusalem and the rest of the Holy Land by first conquering Egypt, ruled by the sultan al-Adil, brother of Saladin. In 1213, Innocent III called for another Crusade at the Fourth Lateran Council, and in the papal bull Quia maior. Innocent died in 1216 and was succeeded by Honorius III who immediately called on Andrew II of Hungary and Frederick II of Germany to lead a Crusade. Frederick had taken the cross in 1215, but hung back, with his crown still in contention, and Honorius delayed the expedition. Andrew II left for Acre in August 1217, joining John of Brienne, king of Jerusalem. The initial plan of a two-prong attack in Syria and in Egypt was abandoned and instead the objective became limited operations in Syria. After accomplishing little, the ailing Andrew returned to Hungary early in 1218. As it became clear that Frederick II was not coming to the east, the remaining commanders began the planning to attack the Egyptian port of Damietta. The fortifications of Damietta were impressive, and included the Burj al-Silsilahthe chain towerwith massive chains that could stretch across the Nile. The siege of Damietta began in June 1218 with a successful assault on the tower.