The author here does not spell out how the Lord routed Sisera and his army. But Judges 5:4 helps us out: Lord, when you sent out from Seir, when you marched from the region of Edom, the earth trembled and the heavens dropped, yes, the clouds dropped water.
The Lord routed Sisera and his army by sending a thunderstorm! That’s why the river Kishon was critically important. It overflowed, and flooded the area! And so Judges 5:21 says, The torrent Kishon swept them away, the ancient torrent, the torrent Kishon.
Sisera’s legendary iron chariots became stuck in the muck. All his tactical advantage went down the drain as Barak’s infantry charged down from Mount Tabor
and absolutely destroyed the Canaanites.1
15 And the LORD routed Sisera and all his chariots and all his army before Barak by the edge of the sword. And Sisera got down from his chariot and fled away on foot.