The city in modern Iran that Genghis Khan’s forces destroyed in 1221 was Nishapur.
Nishapur was one of Khorasan’s major cities, celebrated for its scholars, markets, textiles, pottery, and turquoise trade. It stood on important Silk Road routes and had been a regional capital under several Persian dynasties.
In 1221, Genghis Khan sent his son Tolui to campaign in Khorasan against Khwarezmian strongholds, including Merv, Nishapur, and Herat. Tolui’s army besieged Nishapur after the city resisted Mongol authority. Once the defenses fell, the city suffered a devastating massacre and near-total destruction.
A common mix-up is confusing Nishapur with Merv or Herat, which were also attacked during the Mongol invasion. The destruction of Nishapur was especially remembered because it was connected in contemporary accounts with vengeance for the death of Taghachar, Genghis Khan’s son-in-law. Medieval casualty figures are extraordinarily high and disputed by modern historians, but the city’s destruction itself is well established.