Judges 5:8

8. He chose gods that were new Israel chose new gods; that is, worshipped false gods, the idols of the heathen. See Judges 2:12-13.

Then… war War followed Israel’s idolatry, as a penal consequence.

Forty thousand in Israel This is not to be taken as an exact specification of the whole number of warriors in Israel at this time, but a round number, purposely large, among whom no arms could be found; and the question is a poetical intimation of the great destitution of warlike weapons in Israel. Their oppressors had probably robbed them of their arms, (comp. 1 Samuel 13:19-22,) and they had become too indifferent, or too dejected, to seek for new ones.