Adam Clarke on Judges 11:11

Jepthah went with the elders - The elders had chosen him for their head; but, to be valid, this choice must be confirmed by the people; therefore, it is said, the people made him head. But even this did not complete the business; God must be brought in as a party to this transaction; and therefore Jephthah uttered all his words before the Lord - the terms made with the elders and the people on which he had accepted the command of the army; and, being sure of the Divine approbation, he entered on the work with confidence.