There is no mention of any resistance on the part of the Midianites. The Israelites saw in this and in the preservation of all those engaged, proofs that the Lord had been with them in the work, and hence, the free-will oblation of Numbers 31:50.
Related Commentaries of Numbers 31:49
Numbers 31:49
49 And they said unto Moses, Thy servants have taken the sum of the men of war which are under our charge,d and there lacketh not one man of us.