Rashi and Rashbam (citing the Sifri): It Implies that they had to be handed over by force once they heard that Moshe's life was connected to the battle against Midyan. ¹ And it teaches us the extent with which the leaders of Yisrael (Moshe) are beloved by the people - bearing in mind that, on a previous occasion, Moshe complained "Od Me'at u'Sekaluni!", ² yet when it came to the crunch, they refused to be mobilized and had to be conscripted by force. ³

Targum Onkelos: It means simply that they were chosen'.

Targum Yonasan: It implies that they volunteered to be conscripted.

Ramban #1: Because most of the people, who had sinned with the daughters of Mo'av, were unworthy of avenging Yisrael; so they picked soldiers who were known to the people as Tzadikim ¹ with instructions to destroy Midyan. ²

Ramban #2: Based on the initial command to oppress the Midyanim ³ and the recent one ⁴ to avenge the B'nei Yisrael, Moshe sent a relatively small force to wreak havoc in the towns, to cut down their fruit-trees, stop up their water-wells and to ruin their better-quality fields with stones. And it was only when Hashem miraculously delivered Midyan into their hands that Moshe became angry with the officers, as the Pasuk goes on to describe.

Oznayim la'Torah (in Pasuk 7): To lead the Midyanim to believe that they were bound to win, and therefore decline to take advantage offered to them to flee. ⁵

Rashi #1 (in Pasuk 3) Targum Onkelos and Targum Yonasan: It means 'armed troops'.

Rashi #2 (in Divrei Hayamim 1, 11:24): It means 'skillful troops'.