Mesopotamia was called the crossroads of the world because traders from Europe, Asia, and Africa all met and interacted in this area.
There are two major rivers:

Tigris: Consistent, farmable, trade on, live near
Euphrates: Violent, erratic, feared by people, flooded uncontrollably, killed many people
People living in this area were always in a constant state of defense. Because the geography really had no natural barriers, all they could depend on was their own man made defenses

• North: Flat land
• South: Flat land
• East: Flat land and a river
• West: Mountains existed, but they were not large enough to stop anyone. There was also a river

Before the Greeks, Mesopotamian empires started out as small towns that became larger and larger. Thousands of people would live near these major cities. Most of these cities emerged because the city was either in a really good geographical place, or it was a solid trading city.

The three major city-states of early Mesopotamia were: Ur, Erech, and Kish. Each city-state worshipped their own god. Each city-state believed their god was better than every one else's god. Priest of this time didn't exactly stop them from thinking this. Therefore, city-states were always at war with each other.