Mesopotamian civilization developed between the Tigris and Euphrates Rivers in modern-day Iraq. The Sumerians established the first cities and organized into independent city-states with their own governments. They had an advanced culture with writing, science, and math. Later, the Akkadians and Assyrians formed large empires that ruled the region for hundreds of years, unifying the people but also demanding heavy taxes. The last major power was the Chaldean Empire led by King Nebuchadnezzar, which constructed the famous Hanging Gardens of Babylon before ultimately falling to the Persians.