Then the Spirit of Yahweh came on Jephthah, and he passed over Gilead and Manasseh, and passed over Mizpeh of Gilead, and from Mizpeh of Gilead he passed over to the children of Ammon.
The writer wrote Judges 11:29-32 using a chiastic structure. This section begins and ends with the promise and fulfillment of God giving Jephthah victory. When the Spirit came on him there was no doubt that he would defeat the enemy. The center of the chiasm relates Jephthah bargaining with God to ensure victory. He did not need to make this vow. He had already testified that God had given His people victory in the past (Judges 11:21; Judges 11:24). Apparently his faith was not as strong as it might have been, and this weakness led him to seek a guarantee of success by making the vow.