In 1090, Hasan-i Sabbah, a Nizari Ismaili missionary, seized the mountain fortress of Alamut in the Alborz range of northern Persia through infiltration rather than open assault, establishing a base from which the Nizari Ismailis built a network of fortified strongholds across Persia and later Syria that operated as a genuinely independent state for over a century and a half, surrounded by and frequently at war with the Sunni Seljuk empire and, later, Crusader principalities and Ayyubid Syria. Rather than fielding conventional armies it could not match in numbers, the Nizari state relied on fortified, nearly unassailable mountain positions and on a corps of fida'i, devoted operatives trained to carry out targeted assassinations of hostile rulers and commanders, a tactic that let a small and territorially scattered state deter far larger enemies.
The order's founder was also a serious religious and political thinker within Shia Islam, reorganizing Ismaili doctrine around support for Nizar as the rightful line of imams against the reigning Fatimid caliph in Cairo, a succession dispute that split the wider Ismaili movement and gave the Nizari branch its distinct identity and its own line of imams based first at Alamut and, after the Mongol destruction of the fortress in 1256, in dispersed communities across Persia, Syria, Central Asia and South Asia.
The Mongol conquest under Hulagu Khan ended Alamut's political independence in 1256, dismantling the fortress network and killing much of its leadership and library, but it did not end the Nizari Ismaili faith itself, which survived underground and in dispersed communities and continues today, discussed further in the companion article on the order's legacy and legend.