C. F. Keil and Franz Delitzsch on 1 Samuel 31:7

1Sa 31:7

When the men of Israel upon the sides that were opposite to the valley (Jezreel) and the Jordan saw that the Israelites (the Israelitish troop) fled, and Saul and his sons were dead, they took to flight out of the cities, whereupon the Philistines took possession of them. עבר is used here to signify the side opposite to the place of conflict in the valley of Jezreel, which the writer assumed as his standpoint (cf. 1Sa 14:40); so that העמק עבר is the country to the west of the valley of Jezreel, and היּרדּן עבר the country to the west of the Jordan, i.e., between Gilboa and the Jordan. These districts, i.e., the whole of the country round about the valley of Jezreel, the Philistines took possession of, so that the whole of the northern part of the land of Israel, in other words the whole land with the exception of Peraea and the tribe-land of Judah, came into their hands when Saul was slain.