John Gill Commentary Judges 11:27

"I therefore have not sinned against thee, but thou doest me wrong to war against me: Jehovah, the Judge, be judge this day between the children of Israel and the children of Ammon." — Judges 11:27 (ASV)

Wherefore I have not sinned against thee
Had done him no injury, not wronged him of anything, nor had taken away any part of his country from him; this Jephthah said in the name of all Israel, of whom he was governor:

but thou doest me wrong to war against me ;
meaning that he had no just cause to commence a war against Israel, but acted an injurious part; and seeing things could not be adjusted in an amicable way, but must be decided by the sword, he leaves the affair with the Lord, and appeals to him:

the Lord the Judge ;
the Judge of the whole earth, the omniscient God, that knows all things, the right and wrong of every cause, on which side truth and justice lie:

be Judge this day between the children of Israel and the children of
Ammon ;
not that he expected a decision of the controversy between them would be made that precise and exact day; but that from henceforward the Lord would appear, by giving success to that party which was in the right in this contest.