Muslim regions were no longer unified in any real way, by the time the real Crusaders arrived. I’ve previously explained the three most significant blows to Muslim unity: Fatimid evangelism, North African puritanism, and the rapid conquest of Turks from the east.
In the years running up to European invasion, the Fatimid dynasty’s fervent belief system had fractured into three factions. The Druze refused to accept as Imam any of the Fatimid Caliphs after al-Hakim. Adherents to the esoteric doctrine of Druze theology were, as now, clustered in Syria. An early persecution prompted them to make secrecy a tenet of their faith, so they stayed very quiet for the next centuries.
The next major break was more disruptive. Caliph al-Mutansir, who died in 1094, supposedly wanted his older son, Nizar, to be the next Caliph and Imam. But after al-Mutansir died, the younger son, al-Mustali, seized power. Nizar had to hide.
This event created the next Ismaili split; most Ismaili splits were over which brother was supposed to succeed the father. While the majority of Fatimids in Egypt accepted Caliph al-Mustali (what else could they do?), a radical group of Nizar supporters did not. They fled Egypt, following a trail of believers and safe houses into Persia.
The Seljuks in Persia were deeply hostile to Ismailis and other kinds of Shi’ites. There was no way that Turks could ever be seen as close relatives of Mohammed, so all Shi’ite doctrine was directly hostile to their rule. They persecuted and executed Ismaili missionaries as possible.
By the 1090s, when Caliph al-Mutansir was getting old in Cairo, Seljuk power was growing, mainly under the guidance of a Persian vizier who served several Seljuk generals in succession. Around this time, Hassan i-Sabbah (the chief Ismaili Da’wa in Iran) obtained the remote hilltop fort in northern Iran that became the headquarters for Ismailis and then specifically Nizaris. He named it Alamut, meaning Eagle’s Nest (or Eagle’s Teaching). Once he entered it, he never left. He shut himself into a room and devoted his hours to scholarship and prayer.
The Ismailis wanted to strike back at Seljuk Sunni power; as Iranian Ismailis were few in number, they had to use strategic killing rather than frontal assaults. In Alamut, lower-ranked believers were trained as killers. Their first targets were Seljuks, but after the Nizari split, Alamut considered itself at war with all Muslim rulers. Anyone but Nizar or his descendants was an anti-Caliph who led people straight to hell.
We’ve all heard that the first Assassins used hashish either to evoke Paradise or to carry out their attacks. Current scholarship is against this popular idea, seeing it as a slander put out by non-Nizaris (ie. pretty much everyone else). Ismaili doctrine was extreme and fervent, and it may have been enough to inspire their self-sacrificial assassins, the fedayeen, to give up their lives.
Assassins were trained for stealth, patience, and final delivery of the planned blow. Their weapon was the knife, often with a poisoned tip. They were allowed to take time to assimilate to a town, or to disguise themselves as harmless persons (like whirling dervishes). But eventually, they always got their man. Sometimes their orders were only to threaten and frighten a ruler, but often they carried out public assassinations, like on the steps of the main mosque. Other times, they assassinated in private. The strategic plan was to make everyone else believe that Nizari assassins were almost supernatural in their ability to reach their targets.
The first Alamut-ordered assassination was of the Seljuk vizier, Nizam al-Mulk. He was a gifted ruler who succeeded in maintaining power balance among the last of the Abbasids, various Seljuks, and yet more Turkic tribes flooding into Persia. But while he was traveling from Isfahan to Baghdad on a litter, a Sufi whirling dervish approached him, perhaps for alms. It was really an Ismaili assassin; he stabbed the Vizier. Only our imaginations can suggest what might have happened if the talented vizier had been alive when the Crusaders arrived. Instead, the assassins created a power vacuum.
After Alamut became the Nizari stronghold and headquarters, the Nizaris became the most feared sect. Any murders carried out in the Muslim zone for nearly 200 years were attributed to Nizari assassins. It was often true, but it served their purposes also when it wasn’t. Nizari doctrine spread, and there were other hilltop fortresses to train assassins. Just to glance ahead at the future: eventually, the Mongols destroyed Alamut. Nizaris continued to believe their doctrine but became a dispersed, secretive sect. The Aga Khan is the current Imam of the Nizaris. The Khans of modern times have never tried to seize secular power and do not order assassinations.