**Who:** Ancient civilizations, such as Mesopotamia, Greece, and Rome.
**What:** Independent political entities consisting of a city and its surrounding territory.
**Where:** Found in various regions, including Mesopotamia, Greece, Italy, and other parts of the ancient world.
**When:** City-states emerged in different time periods, with Mesopotamian city-states appearing around 3000 BCE, Greek city-states in the 8th century BCE, and Roman city-states in the 6th century BCE.
**Why:** City-states provided self-governance, protection, and cultural identity to their inhabitants, fostering trade, innovation, and competition among neighboring city-states.