 Saladin launched his last offensive against Mosul in late 1185 , hoping for an easy victory over a presumably demoralized Mas 'ud , but failed due to the city 's unexpectedly stiff resistance and a serious illness which caused Saladin to withdraw to Harran . Upon Abbasid encouragement , Saladin and Mas 'ud negotiated a treaty in March 1186 that left the Zengids in control of Mosul , but under the obligation to supply the Ayyubids with military support when requested . 
