Albert Barnes Commentary 1 Samuel 11:1

"Then Nahash the Ammonite came up, and encamped against Jabesh-gilead: and all the men of Jabesh said unto Nahash, Make a covenant with us, and we will serve thee." — 1 Samuel 11:1 (ASV)

Nahash was king of the Ammonites, as is clear from 1 Samuel 12:12. He seems to have been connected with the family of David, since David’s sister Abigail was “the daughter (perhaps granddaughter) of Nahash” (2 Samuel 17:25; 1 Chronicles 2:16–17). Perhaps because of this connection, he and his family were very friendly to David (2 Samuel 17:27).

Jabesh-Gilead must have been repopulated after its destruction. The Ammonites and Moabites resented the possession of Gilead by the Israelites (Judges 10:6–18; Judges 11).