Judges 12:2

2. When I called you Jephthah speaks in the name and as representative of the eastern tribes. These tribes were the ones specially afflicted by Ammon, and when they could no longer endure the oppression they resolved on fight. See Judges 10:17-18. Then they looked for a leader; and then it doubtless was they appealed to the western tribes, and especially to Ephraim, to come and save them from their bitter foes. Jephthah himself, too, when collecting his forces to go against the Ammonites, probably called on them to come and help in the war. Keil thinks the Ephraimites probably refused their co-operation because the eastern tribes had appointed Jephthah as commander without consulting them. The designed brevity of the history led to the necessary omission of many minor facts and details.