Jamieson-Fausset-Brown on Numbers 31:3
Arm some of yourselves--This order was issued but a short time before the death of Moses. The announcement to him of that approaching event [Num 31:2] seems to have accelerated, rather than retarded, his warlike preparations.
John Gill on Numbers 31:4
Of every tribe a thousand,.... So that the whole number of those that were armed were 12,000 as after given:
throughout all the tribes of Israel; this is observed, as Jarchi thinks, to comprehend the tribe of Levi, which in some cases was left out of the account:
shall ye send to the war; to fight with Midian.
John Calvin on Numbers 31:3-6
And Moses spake unto the people. There is no doubt but that Moses delivered the commands which he had received from God; although, therefore, it is stated [205] that only ten thousand went forth to the war, yet the facts themselves demonstrate that the number, as well as the mode of warfare, was prescribed by God. And assuredly it would have been inconsiderate of Moses to attack so great a people with so small a band; and thus he would have deservedly incurred the penalty of his rashness, if he had attempted it of his own accord; still, when God's command had preceded, he happily concluded the matter, which had been properly and rightly undertaken. Nor can it be questioned but that God desir…
Read the whole note in the consoleKeil & Delitzsch on Numbers 31:3-6
To carry out this revenge, Moses had 1000 men of each tribe delivered (ימּסרוּ, see at Num 31:16) from the families (alaphim, see Num 1:16) of the tribes, and equipped for war; and these he sent to the army (into the war) along with Phinehas the son of Eleazar the high priest, who carried the holy vessels, viz., the alarm-trumpets, in his hand. Phinehas was attached to the army, not as the leader of the soldiers, but as the high priest with the holy trumpets (Num 10:9), because the war was a holy war of the congregation against the enemies of themselves and their God. Phinehas had so distinguished himself by the zeal which he had displayed against the idolaters (Num 25:7), that it was imposs…