Mongke Khan's brother Hulagu set out to besiege the Assassins in their primary fortress at Alamut.
The sect leader who ordered the attack on Mongke had been killed by his own followers for drunkenness, and his rather useless son now held power.
The Mongols threw all of their military might against Alamut, while also offering clemency if the Assassin leader would surrender.
On November 19, 1256, he did so. Hulagu paraded the captured leader in front of all the remaining strongholds, and one by one they capitulated.
The Mongols tore down the castles at Alamut and other places, so that the Assassins could not take refuge and regroup there.
The following year, the former Assassin leader asked permission to travel to Karakoram, the Mongol capital, in order to offer his submission to Mongke Khan in person.
After the arduous journey, he arrived but was denied an audience. Instead, he and his followers were taken out into the surrounding mountains and killed.
It was the end of the Assassins.