The Aztec Empire expanded under the leadership of Moctezuma to cover much of Central Mexico and Guatemala by the 15th century, with a population of over 5 million people. The Aztecs established tributary city-states rather than forcing cultural assimilation, demanding food, goods, and labor as taxes. Successful warfare was crucial to the Aztec religion and empire, allowing the acquisition of lands, resources, and sacrificial victims to please the god Huitzilopochtli. While local rulers retained autonomy, heavy-handed punishment of tax rebellions undermined unity in the far-reaching but loosely-controlled Aztec domain.