One of the themes that runs through much of human history is the tendency toward ever larger agglomerations of people under the rule of a single government. Human groupings began small with bands of foragers and hunters. With agriculture came the rise of villages and concentrations of farming settlement. Civilization, first emerging in Mesopotamia, brought with it the first city states and kingdoms. But it was the conqueror Sargon of Akkad who created the first empire sometime in the 24th century BCE. Much larger than any previous jurisdiction, Akkad absorbed numerous city-states and kingdoms of Mesopotamia under a single ruling house. But in time, the Akkadian Empire (see left above) would be eclipsed by far larger entities, such as the Persian Empire (see right above) of the sixth century BCE and ultimately the Roman Empire (see left below) from the late first millennium BCE and the early first millennium CE. Other parts of the world saw empires emerge independently, such as the Aztecs of the period from 1200 to 1500 CE (see left below), themselves falling to and being absorbed by the far larger Spanish Empire in the early 16th century.
Two things characterize an empire as opposed to a city-state or kingdom, and neither is necessarily about size. While the Roman Empire was vast, the Aztec Empire merely encompassed a small slice of the modern-day country of Mexico. Instead what makes empires unique is, first, that they encompass different cultures and societies. Secondly, they are marked by flexible borders and seemingly insatiable appetites to expand them.