The increased economic and military power of the Sumerians can be attributed to several factors, among which the most relevant are:
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The development of bronze to make weapons: The Sumerians were among the first to develop and use bronze, which significantly enhanced their military capabilities through the production of stronger and more effective weapons.
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A standing army of many trained soldiers: The establishment of a professional, trained army allowed for better organization and effectiveness in military campaigns and defense.
While the formation of alliances with other city-states could also contribute to their power, it is less direct in enhancing their military and economic strength compared to the advancements in weaponry and the establishment of a standing army.
Thus, the most definitive contributions would be the development of bronze for weapons and the existence of a standing army of trained soldiers.