Which of the following contributed to the increased economic and military power of the Sumerians?

a standing army of many trained soldiers

the formation of alliances with other city-states

a series of highly strategic army leaders

the development of bronze to make weapons

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The answer is: the formation of alliances with other city-states

Explanation:

One of the ancient civilizations that was born in Mesopotamia was the "Sumer." The people under this civilization were called "Sumerians." The formation of alliances with other city-states contributed to the increase economic and military power of the Sumerians. These states were united under a king and their alliance made them stronger. A single king led them into war such as the case of "Enembaragesi of Kish" who successfully led the people into victory by winning against the Elamites. This was also the reason why it was so hard to conquer the Sumerians and to have a full control over their city-states. So, this increased not only their economic power but also their military power.

Thus, this explains the answer.