The War Against Benjamin
The grisly evidence of the crime of the Gibeahites produced shock in the nation of Israel. A council was held at Mizpah, the Levite giving his testimony as to what had happened. All Israel resolved to take action against the Gibeahites.
A delegation was sent to the Gibeahites demanding the surrender of the "sons of Belial" (a term denoting wicked, worthless, perverse individuals). But when the Gibeahite elders showed themselves to be implacable, the situation became ominous. Indeed, all Benjamin rallied to the aid of Gibeah. The Benjamites fielded an army of 26,000 men against 400,000 soldiers out of the remaining tribes.
That the men of Benjamin would determine to fight the other 11 tribes appears remarkably senseless, even though they were known for their courage and military prowess. Genesis 49:27 hints at this and 1 Chronicles 8:40 and 12:2 provide examples. Judges 20:16 states that their army included 700 men who possessed devastating power by use of the sling (the same weapon with which David later slew Goliath). It was an effective weapon: "The sling, which was employed with a left-handed motion, must not be confused with a modern schoolboy's catapult; it was a formidable weapon of war used in the Assyrian, Egyptian and Babylonian armies as well as in Israel.... It has been estimated that stones weighing up to one pound could be projected with uncanny accuracy at speeds up to 90 m.p.h.!" (Tyndale Old Testament Commentaries, note on verses 15-16).
With the clash between the two armies looming, the Israelites obtained counsel from God on the matter and, after initial reverses, completely routed the Benjamites.
We do not really know the reason that God initially allowed the Israelites to suffer 40,000 casualties with virtually no Benjamite casualties before giving the Israelites any help. There may have been tactical reasons for the lopsided nature of the first engagement. The Tyndale commentary on Judges offers this observation: "The hilly terrain in the vicinity of Gibeah favored a defensive force rather than an attacking force, especially if the former was in a strong position, as was likely in this case, since the Benjamites were familiar with their tribal portion. In such a situation superior numbers were of limited value, since they could not be effectively deployed, and a determined group of men armed with slings could inflict heavy casualties on an attacking force.... [And] in the battle which ensued the psychological advantage lay with the Benjamites. They would fight desperately because they were fighting for their lives, whereas the opposing force, while convinced of the rightness of their cause, may have had little heart to engage in a civil war" (note on verses 19-25). If this analysis is correct, it is an interesting parallel to the American Civil War, in cases where southern armies overwhelmed numerically superior armies of the north.
Perhaps more importantly, God may not have been especially happy with the other tribes (that their hearts were not really right is evident in what happened in the aftermath of the war). We do see that they were driven to fasting and sacrificing before God, something quite rare in this period. Perhaps God wanted them to see the need for this. In any case, the Israelites finally succeeded using a tactic similar to that used at Ai. All but 600 Benjamite men were slaughtered in the fighting. The 600 men fled to a stronghold and maintained themselves there for four months.
But during that four months, the Israelites did something just as unthinkable as the crime that sparked the war in the first place—they went through Benjamin's territory and slaughtered the entire tribe, women and children, young and old. This was an unjustified atrocity, though the Israelites may have considered it just retribution because the Benjamite cities they butchered had sent forces to aid the wicked men of Gibeah. In any case, it was an instance of anger and revenge taking precedence over self-control. When the slaughter was complete, only the 600 men in the stronghold survived.