With Ögedei's consent to launch a campaign, Chormaqan left Bukhara at the head of 30,000 to 50,000 Mongol soldiers. He occupied Persia and Khorasan, two long-standing bases of Khwarazmian support. Crossing the Amu Darya River in 1230 and entering Khorasan without encountering any opposition, Chormaqan passed through quickly. He left a sizable contingent behind under the command of Dayir Noyan, who had further instructions to invade western Afghanistan. Chormaqan and the majority of his army then entered Tabaristan (modern-day Mazandaran), a region between the Caspian Sea and Alborz mountains, in the autumn of 1230, thus avoiding the mountainous area to the south, which was controlled by the Nizari Ismailis (the Assassins).