Why did the Mongols sometimes spare people who surrendered voluntarily?

Under Genghis Khan, the Mongols offered voluntary surrender as a strategic policy to avoid the costs of prolonged conflict and to expand their military capabilities. When a population surrendered, they were spared, and their soldiers or tribesmen were often incorporated into the Mongol forces as federates, helping to bolster the ranks of an army that was often composed of only a minority of ethnic Mongols.1

This practice served as a form of psychological warfare; by offering peace in exchange for submission, the Mongols sought to sap enemy resistance and minimize the need for costly sieges. Conversely, those who resisted faced wholesale slaughter or enslavement, a tactic used to foster terror and discourage further opposition. While surrender generally ensured safety, it was not an absolute guarantee, as the Mongols remained pragmatic and occasionally ordered massacres for tactical reasons even after a city submitted. Ultimately, this approach allowed the Mongols to maintain the vigor of their own people while effectively utilizing the skills and manpower of conquered populations to build their empire.12