In the final years of the Prophet Muhammad's (peace and blessings upon him) life, he conquered the city of his birth, Mecca.
If you look at the actions of most conquerors throughout history, you'll find anything but mercy. Conquerors are typically quick to exert their power over the conquered people as a display of authority.
But that's not what happened when the Prophet Muhammad (peace and blessings upon him) conquered Mecca.
Despite the fact that he was driven out of the city just years prior, the repeated acts of aggression initiated by the Meccans after the Muslims had fled to Madina, and the violation of their peace treaty by the Meccans — the Prophet Muhammad (peace and blessings upon him) dealt with the people of Mecca with nothing but mercy, compassion and gentleness.