Charles Ellicott Commentary Numbers 31:3

"And Moses spake unto the people, saying, Arm ye men from among you for the war, that they may go against Midian, to execute Jehovah`s vengeance on Midian." — Numbers 31:3 (ASV)

Arm some of yourselves ... — Better, Arm from among you (or, from those with you) men for the war. The details of the selection are contained in the next verse.