What weakened the cities of Sumer and led to their conquest?

Climate change was the main factor that diminish the cities of Sumer, it is believed that a long term drought which affected the agricultural production. Agricultural production fell due to the acquisition of grains that tolerated a higher salinity. Water from the Mesopotamian rivers were heavily loaded with salts from the mountains, this water was poured into the fields to be soaked up by the soil or evaporated leaving the salt. As more salt accumulated productivity collapsed. As a result of penurious crop production, Sumer became less capable of responding to famine.

Sumer city-states were, also, eventually absorbed into the Akkadian population. A great inconvenience for Sumer was neighboring the Akkadian Empire, which had an army and a desire for conquest. While Akkadian kings had governmental and administrative experience, the Sumerians had difficulty controlling their empire. Revolts became a common occurrence. Being the first civilization, there was no role model to attend in controlling a large number of communities who were fighting with each other. The only origin of order at the time was the Temple for worshiping the multiple gods they believed in. The temple priests were more worried with collecting the tribute for the gods than with the constant conflict among the communities; and because of this they never set up an army to maintain the order.

The Akkadians were behind the destruction of Mesopotamia's capital city, Ur, around 2000 B. C. The last Sumerians were absorbed into the population and Sumer Became history.