The Mongol horde displayed unparalleled ruthlessness in Eastern Iran: The garrison at Merv was only about 12,000 men, and the city was inundated with refugees

The Mongol horde displayed unparalleled ruthlessness in Eastern Iran: The garrison at Merv was only about 12,000 men, and the city was inundated with refugees from eastern Khwarezmia. For six days, Tolui besieged the city, and on the seventh day, he assaulted the city. However, the garrison beat back the assault and launched their own counter-attack against the Mongols. The garrison force was similarly forced back into the city. The next day, the city's governor surrendered the city on Tolui's promise that the lives of the citizens would be spared. As soon as the city was handed over, however, Tolui slaughtered almost every person who surrendered, in a massacre possibly on a greater scale than that at Urgench.