Charles Ellicott Commentary Judges 11:5

"And it was so, that, when the children of Ammon made war against Israel, the elders of Gilead went to fetch Jephthah out of the land of Tob;" — Judges 11:5 (ASV)

When the children of Ammon made war. —The allusion is to some special threat of invasion (acriter instantibus, Vulgate) at the close of the eighteen years of oppression (Judges 10:9).

To fetch Jephthah. —Because by this time he had made himself a great name as a brave and successful chieftain of marauders, who would doubtless come with him to lead the Gileadites.