Charles Ellicott Commentary 1 Samuel 31:7

"And when the men of Israel that were on the other side of the valley, and they that were beyond the Jordan, saw that the men of Israel fled, and that Saul and his sons were dead, they forsook the cities, and fled; and the Philistines came and dwelt in them." — 1 Samuel 31:7 (ASV)

On the other side of the valley. —The words “on the other side of the valley” denote the country opposite the battlefield in the Jezreel Valley, where the writer supposes himself to be standing, the land occupied especially by the tribes of Issachar, Zebulun, and Naphtali. The expression “on the other side of Jordan” is the usual phrase for the country east of the River Jordan. It is highly probable that the alarm caused by the great defeat of their king caused many of the inhabitants in the smaller cities and villages east of Jordan to hastily abandon their houses rather than be exposed to the insolence and demands of the invading army.

Still, the Philistine army in this direction could not have penetrated very far, as, shortly after Gilboa, we hear of Abner rallying the friends of the house of Saul around Prince Ishbosheth, whom he proclaimed king at Mahanaim, a town some twenty miles east of the river. The country to the south of the Jezreel Plain does not appear to have been overrun by the victorious army. The presence of David in that part no doubt ensured its immunity from invasion.