“ And I will draw to you, to the river Kishon, Sisera, the captain of Jabin's army, with his chariots and his large body of fighting men, and I will deliver him into your hand.”
These were the words of their battle general, Yahweh. Once Sisera heard of their gathering on Mount Tabor, in what was clearly an attempt at rebellion, he would take his chariots and men over to the mount in accordance with Yahweh's plan. Then Yahweh would arrange for them to be delivered into the hands of Barak's small army.
But the song of Deborah makes clear that Barak had reserves to call on from the tribal confederacy. Some came from Ephraim, others from Benjamin, and more from Machir (Manasseh) (Judges 5:14).