The Assassin Order used a network of assassins and spies to expand their political influence during medieval times in Syria and Iran. They used a form of unconventional warfare to intimidate foreign leaders to submit and political appease them.
The Mongolians destroyed the Assassins through military force.
This is a summary of the order by Hodgson:
Following the death of the Fâtimid caliphate al Mustansir in 1094, members of a faction in Persia that supported a deposed claimant to the caliphate, Nizâr, believed they now represented Fâtimid interests. These Nizârî Ismâî’lîs ended up separating themselves from mainstream Islam and creating their own state in parts of present-day Syria, Iraq, and Iran. In order to establish and maintain regional control, the Nizârî Ismâî’lîs used political murders and spies to subjugate or influence rival caliphates and the dominant Saljûqs.
Instead of looking at the historical details, I will analyze this through abstraction to try and explain the agent operations.
The Assassins sent in undercover agents who infiltrated the entourage of an emir or lord. The Assassins made political demands of this emir and if he did not meet these demands their assassin would kill him.
The Arab, Turkic, and Iranian Emirs had three general responses to these demands. They could 1) use military force to attack Assassin territory and strongholds; 2) use police to uncover assassins; or 3) politically appease the Assassins.
Interestingly, this tactic was largely ineffective against the Europeans. The Europeans had a robust command structure, so when a leader died, another rose to take his place without changing the political structure. It was partially merit based, so leaders were somewhat interchangeable. Arab leaders based their power on a cult of personality, so each ruler was unique in influence and behavior.
Look at the Emir’s three choices:
Policing was ineffective Police captured a few assassins but many slipped through and killed emirs. Short of cutting off a city or fort from the outside world, there was always a chance disguised assassins would penetrate a ruler’s defenses.
Military force could work but was extremely risky. While the assassins lacked military power to withstand a serious offensive, Emirs lacked the time to mobilize their armies. A military campaign would take months or years, in which time, an assassin would kill the emir and halt the offensive.
So there were two counters: Appease the Assassins or militarily destroy them. Most Persians did not wish to risk their own lives, so they choose appeasement.
Appeasement offered the greatest benefit and the least risk of loss over the short-term. The Assassin Order intimidated individual emirs with the threat of assassination. However, appeasement weakened over the long-term. The Assassins added additional demands forcing the emirs to favor military action. Appeasement routinely broke down into new waves of assassinations.
The Mongols found the solution to the Assassin cult, and it was as effective as most Mongolian solutions to problems.
The Khan used a mixed strategy. Policing was an ineffective counter, but it had some utility for lowering the risks of assassination over the short term. This purchased the time needed to wage an effective military campaign.
When the Assassins threatened the Khan, he ordered his police and bodyguards form an elaborate security network so everyone had to pass multiple checkpoints to meet him. Eventually, an assassin would have penetrated the defenses. The khan’s real goal was to bring his army to the Assassin fortresses and exterminate them before an assassin reached him.
The Mongolians attacked the Iranian Assassin Order, captured its strongholds, and executed all its members.