12. And the children of Reuben, etc He makes mention of the expedition of the two tribes and half tribe, as they did not set out to engage in warfare on their own private account, but to assist their brethren, by whose valor their own possession had been obtained in seizing the land of Canaan. Moses had laid them under this obligation, and they had bound themselves by oath that they would accompany the rest of the people till all should have obtained a quiet settlement.
They again made the same promise when the camp was about to be moved as we saw in Joshua 1. But from the narrative here we gather that only a part was selected, for the number amounts only to forty thousand, that is, a third, or about a third of the number ascertained by the census taken shortly before. Now, as they are everywhere said to have performed their promise, it may be probably conjectured that it was not the intention of Moses strictly to insist that all who had assented should leave their wives and children, and do military service in the land of Canaan till it was wholly subdued. And certainly it would have been harsh and cruel to leave an unwarlike multitude unprotected in the midst of many hostile nations. Nor would the remains of the enemy, assisted by neighboring nations, have long failed to take advantage of such an opportunity to avenge themselves by massacring the women and children. It was necessary, therefore, in a country not yet sufficiently pacified, permanently to retain a force sufficient to prevent incursions. Moses was not of so stern a nature as not to consult for the helpless. Nay, his prudence and equity would never have allowed him to leave a territory lately seized by arms unoccupied by a body of troops.
We may add, that such an immense concourse would have impeded rather than assisted the acquisition of the land of Canaan. All which Moses required, therefore, was simply that the Reubenites and Gadites should not, while their brethren were engaged in carrying on the war, remain indolently at home and eat their food at ease without giving any assistance to those to whom they were indebted for having obtained the inheritance. And the good faith of the forty thousand was approved by their not declining the burdens, toils, and perils of warfare, while the remainder of their own tribes were enjoying quiet. They might readily have alleged that they were as well entitled as the others to exemption, but in proceeding with alacrity after the levy was made, to obey the orders given them, without envying the immunity given to their brethren, they show that they were voluntarily and heartily disposed to do their duty. At the same time, it is not doubtful that by accepting the flower of their tribes, the handle for complaint and quarrel was cut off. For it could not justly have been maintained that not even the aged and worn out, or the young and feeble, were to be spared. Some, perhaps, may be inclined to conjecture that the army was raised not by choice but by lot, though it rather seems to me that all who were most robust and best able to bear fatigue were enrolled.