So the tribe of Benjamin got what they deserved for this heinous crime. But not without a couple of false starts.

Why did God allow the Benjaminites to win the first couple of battles?

Well, when we look closely at the narrative here, we see that in the first case, the men of Israel did not consult God as to how they should address the problem. They decided on their own that they would go to war against the Benjaminites. They didn’t ask God.

It wasn’t until they were deciding who would go first against Benjamin that they consulted Him.

They were so sure about the fact that they were fighting a righteous battle that they assumed God’s blessing. They didn’t ask.

So on the first day of battle, 22,000 of Israel’s men perished.

Then, they got reinforcements, formed their battle line, and then cried before the Lord until evening and then asked if they should continue the fight against Benjamin. And God replied, “Go fight them.”
And 18,000 more men of Israel died.

Then, look at what they did. They all went to Bethel, which means “The House of God”. They went to the temple, where the Ark of the Covenant was, which represented the very presence of God. There, they sat and cried in His presence, and they fasted. And they sacrificed burnt offerings and fellowship offerings. And after all this, they asked, “Should we continue to fight, or should we stop?” And then God said, “Go! Tomorrow I will hand them over to you.”

The difference was that in the final attempt at defeating the tribe of Benjamin, the men of Israel were finally at the point of depending completely on God’s leading. The were at their wit’s end, which is where God always wants us. Not that He wants us desperate, but that we acknowledge the fact that our wits are not sufficient. God wants us to humbly come to Him with an attitude of reliance on Him.
Paul said it very well in 2 Corinthians 12:10: “For when I am weak, then I am strong.”