Judges 11:11

‘ Then Jephthah went with the elders of Israel, and the people made him head and commander-in-chief over them, and Jephthah spoke all his words before Yahweh in Mizpah.'

Satisfied with their oath Jephthah went with them, and no doubt took his men with him, promising them due reward. They would form his spearhead attack. Then he was appointed head and commander-in-chief by acclamation of the people and in the presence of Yahweh by an oath. This was done at Mizpah where the Gileadite forces were gathered, somewhat fearful at the thought of the approaching enemy (Judges 10:17). ‘Before Yahweh.' It may well be that the Ark had been brought there to lead them into battle as in Judges 20:27 (compare also 1 Samuel 4:3; Joshua 6:6; Numbers 10:35). Or the oath may have been made at some recognised holy place.