This statement indicates that even though Israel had superior numbers and tactical advantage, they may still have underestimated the determination of the Benjamites. The battle is heavy; victory is not simply a matter of course for the Israelites. The Benjamite army is faced with a life or death struggle, and so their counterattack is intense.
34 And there came against Gibeah 10,000 chosen men out of all Israel, and the battle was hard, but the Benjaminites did not know that disaster was close upon them.