By 1237, the Mongols had conquered most of Central Asia. All of Persia had fallen except for the strongholds of the Assassins - perhaps as many as 100 mountain fortresses.
The Assassins had enjoyed a relatively free hand in the region between the Mongols' 1219 conquest of Kwarezm and the 1250s.
The Mongols were focusing elsewhere, and ruled lightly. However, Genghis Khan's grandson Mongke Khan grew determined to conquer the Islamic lands by taking Baghdad, seat of the caliphate.
Fearful of this renewed interest in his region, the Assassin leader sent a team to kill Mongke. They were supposed to pretend to offer submission to the Mongol khan, and then stab him.
Mongke's guards suspected treachery and turned the Assassins away, but the damage was done. Mongke was determined to end the threat of the Assassins once and for all.