Civilizations arose at different times in different parts of the world. Most grew out of agricultural settlements in valleys of such rivers as the Tigris and Euphrates in Mesopotamia, the Nile in Egypt, the Indus in South Asia, and the Huang He in China. In a typical situation, villages would band together to form small kingdoms. They would later unite under a single monarch who was both a political and religious leader. As their political and military power grew, rulers would try to expand the frontiers of their territories to create empires. Although ruling power was usually hereditary, governments were often overthrown, either by rebels within the country (as in China) or by invaders (as in Mesopotamia).