Adam Clarke on Judges 20:18

Went up to the house of God - Some think that a deputation was sent from Shiloh, where Phinehas the high priest was, to inquire, not concerning the expediency of the war, nor of its success, but which of the tribes should begin the attack. Having so much right on their side, they had no doubt of the justice of their cause. Having such a superiority of numbers, they had no doubt of success. See the note on Jdg 20:1.

And the Lord said, Judah - But he did not say that they should conquer.