 Complete the following phrase:  After the Mongol invasion of the Khwarazmian Empire, many Khwarazmians survived by employing themselves as mercenaries in northern Iraq. Sultan Jalal ad-Dins followers remained loyal to him even after his death in 1231, and raided the Seljuq lands of Jazira and Syria for the next several years, calling themselves the Khwarazmiyya. Ayyubid Sultan as-Salih Ayyub, in Egypt, later hired them against his uncle as-Salih Ismail. The Khwarazmiyya, heading south from Iraq towards Egypt, invaded Crusader-held Jerusalem along the way, on 11 July 1244 Siege of Jerusalem 1244. The citys citadel, the Tower of David, surrendered on 23 August, and the Christian population of the city was expelled. This triggered a call from Europe for the Seventh Crusade, but the Crusaders would never again be successful in retaking Jerusalem. After being conquered by the Khwarazmian forces, the city stayed under Muslim control until 1917, 