From 1210 Hassan III (the Great) of Alamut became leader of the Assassins. During his reign several changes were made to the order, allowing it to survive during the coming Mongol invasion.
Following past attempts on the life of Saladin, the Abbasid Caliphate waged a war with the Assassins. Having failed in attacking Masyaf, they decided to attack the other castle in Alamut. The past rulers were able to repel the Muslims, but they maintained an important presence in Iran, preventing any Assassin expansion. A sort of stalemate was created, as the Muslims could not take Alamut, but the Assassins could not expand into Iran, isolating them at the castle.
In 1210, Hassan became Grandmaster of the Assassins, ruling from the fortified castle in Alamut. Hassan wanted to defeat the Muslims, and sent his assassins out undercover. Taking to the streets of villages and cities in Iran, Hassan began to recruit peasants. He also created influence by installing Assassin-friendly merchants in markets, allowing the Assassins to control sales. The Assassins started riots and revolts across Iran, led by the peasants. This severely weakened Abbasid control over the area. The Abbasids were powerless to do anything, as they were occupied with fighting with the Crusaders in Egypt.
In 1218, with a powerful peasant army, the Assassins layed sieged to Ray, a powerful Abbasid stronghold. After several weeks of sieging the city, it fell and the Assassins took control. Faced with possible attacks coming from the east, the Caliphate had to make a peace agreement with the Crusaders, giving control of Egypt to them. Crusader states were established in Cairo and Alexandria. Without the war, the Abbasid began a campaign to retake Iran.
War in Iran
The Abbisids led an army of 15,000 strong into Iran, with the goal of taking back Ray. The Assassins trained the peasants to fight, and several pitched battles were fought. While the armies fought the Assassins raided Abbisid encampents, disrupted supply lines, and launched ambushes. Hassan also began to send the his assassins out on missions to kill opposing generals. The Abbisids marched on despite these setbacks, and soon covered a large area. The peasants were successful in delaying the Abbisids, which allowed the Assassins to build fortifactions around Ray. With 7,000 troops left, the Abbisids attacked Ray. The fortifications kept the Muslims out of the city, with Assassin archers cutting troops down. During the day, the battle roared but the Abbisids could not enter the city. At night the Assassins raided camps, and took out troops.