This is another story like Ehud that validates the Bible’s accuracy through an unsavory assassination. There is also some subtle direct action from God mentioned in Deborah’s song.
The Canaanite’s of the northern plains were too militarily powerful to be driven out of the land after the land had been divided and mop up duties given to the individual tribes. The Canaanites had grown in the last 100 or so years and now had 900 iron chariots. We don’t hear of the Ark of the Covenant being brought forward, so God was not directly invited to participate in the battle. We do know that Sisera fled the battle on foot. Why would he do that if he had a chariot? In Deborah’s song we hear about rain. The battlefield was in a wide valley where the quick and armored chariots would have an advantage. God sent rain that ran down the mountainsides into the valley creating a bog. The heavy iron chariots got stuck in the mud, making the Canaanites vulnerable to the Israelite foot soldiers.
Jael gives Sisera milk when he asks for water. This explicit detail has always baffled me. I guess it was to make him sleepier. In any case Jael was able to kill the Canaanite king and give Israel a definitive victory. She really nailed it!