John Gill Commentary Judges 11:30

"And Jephthah vowed a vow unto Jehovah, and said, If thou wilt indeed deliver the children of Ammon into my hand," — Judges 11:30 (ASV)

And Jephthah vowed a vow unto the Lord
Before he set out for the land of the children of Ammon, and to fight with them; hoping that such a religious disposition of mind would be regarded by the Lord, and be acceptable to him, and he should be blessed with success in his enterprise:

and said, if you shall without fail deliver the children of Ammon into
my hands ;
though he was assured of the justness of his cause, and of his call to engage in it, he seems to have some little diffidence in his mind about the success of it; at least, was not fully certain of it.