Jamieson, Fausset and Brown on Judges 12:3

3. when I saw that ye delivered me not, I put my life in my hands--A common form of speech in the East for undertaking a duty of imminent peril. This Jephthah had done, having encountered and routed the Ammonites with the aid of his Gileadite volunteers alone; and since the Lord had enabled him to conquer without requiring assistance from any other tribe, why should the Ephraimites take offense? They ought rather to have been delighted and thankful that the war had terminated without their incurring any labor and danger.