Which is a reason Sumer declined as a civilization? Sumerian city-states fought each other. Sumerian rulers outlawed religion. The climate changed drastically. The Tigris and Euphrates rivers dried up.

"Sumerian city-states fought each other" would be the best option from the list, since this internal division led to a collapse of power that left people without protection or a solid economy.

I believe the answer is: Sumerian rulers fought to gain territory

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Because of the conflict that happened between sumerian rulers, The territory of the Sumerian empire started to be divided/separated into smaller empires. This make it much more easier for opposing empire or barbarian to conquer them and pick them apart, which lead to the decline of Sumer civilizations.