What does Judges 7:19 mean?

The plan is in place, and the offensive against the vast Midianite camp has begun (Judges 7:15–18). Gideon and his 300 men walk down to the valley and position themselves in three groups of one hundred men. They spread out at strategic points surrounding the perimeter of the enemy camp. The men will wait for a signal, then reveal torches, blow trumpets, and shout nearly in unison.

In that era, most armies observed three four-hour watches during the night. Scholars suggest these watches were typically set around 6 p.m., 10 p.m., and 2 a.m. Gideon and his men would have been in their places around the camp by about 10 p.m. What happens next occurs "just after" the setting of the new watch. That the book of Judges mentions the exact timing of the attack is no accident: it's a crucial part of Gideon's strategy.

At the moment Israel's men make their move, Midianite guards at the outskirts of the camp are caught unaware. Guards returning from duty are walking through the camp—armed—and in the dark. The rest are likely asleep. The sudden sound of battle horns and torches would cause confusion; it would appear a huge force was bearing down. This would lead men jarred out of sleep to assume the armed figures they saw were enemy troops. As they attack these unrecognized allies, others would see the attack, and assume the aggressors were the enemy. In this way, the entire Midianite force would be thrown into chaos.