In the 3,000 b. c., sumerian farmers realized that the grain surplus they produced could be used as barter for things they did not have. therefore, the sumerians obtained copper from sinai desert traders who were located several hundred miles to the west in order to make weapons and repel nomadic raiders. the sumerians engaged in

The correct answer is - trade.

The Sumerians engaged in trade with their move of exchanging their surplus food for materials like the copper that they needed to develop their weapons and thus their army forces. This was a very smart move from their side because they managed to use the resource they had in abundance but was scarce in the other surrounding areas so that they take what they need to become stronger and pretty much defend more efficiently from the same people they were engaging in trade with.  

it was displayed because they had a love for war