Bible Notes Online - Judges 20 - ESV
Commentary

v1: Israel, for once, was united. Yet Gibeah was probably not alone in its sin.

v7: The Levite recounted his story (v4-6), and pressed the Israelite leaders to make a decision; see also 19.30. 'What should our response be?'

v8-11: Israel gathered to attack Gibeah; this seems to be the decision of the whole nation. One tenth were set apart to gather provisions. Their motivation was to repay the wickedness of Gibeah; no doubt this was what the Levite intended.

v12-14: The opportunity to repent, and deliver up the guilty The tribe of Benjamin refused to deliver up the guilty men for punishment. This is of great concern, was they were seen to condone their sin.  Indeed, they gathered to fight against the Israelites.

v18-28: Israel sought the counsel of God, which was given (v18,23), yet on both occasions, Israel suffered, in total, the loss of 40,000 men. It may have been that Israel was whole was not innocent of wrong doing. Further, the nation had to learn that, to attack another tribe was a serious matter, and such conflict must bring suffering to 'both sides'. Their prayer was about attacking "our brothers" (v23).

After two failed attacks, the Lord God promised victory (v28). Israel were doubtless encouraged to continue the attack. Often victory only comes with real cost, and persistence and suffering. In our personal lives, we may suffer defeats over sin before victory is won.

Above all, God's ways are not our ways; we see but a small part of the whole picture, and we cannot see all 'reasons' and 'causes' for events.

v29-36: Victory was secured with an ambush (v29,33; compare Josh 8.4,19). The defeat was severe, with 25,100 of an original 26,700 killed. The city itself was attacked (v37-39).

v47: Only 600 Benjamites escaped, as Israel routed the army, and put the towns to the sword.