2 Samuel 8:13

And David gat him a name, he gained renown, his fame spread far and wide, when he returned from smiting of the Syrians in the Valley of Salt, being eighteen thousand men, namely, of the Edomites. It seems that the children of Edom took the opportunity offered by David's absence in the Syrian country to make an attack on Southern Canaan. They had advanced to the Southern end of the Dead Sea when the army of David, just back from the Syrian war, fell upon them with such disastrous results.