John Calvin Commentary Numbers 32:20

"And Moses said unto them, If ye will do this thing, if ye will arm yourselves to go before Jehovah to the war," — Numbers 32:20 (ASV)

And Moses said to them. Moses might seem to err on the side of excessive good-nature, because he extends the boundaries prescribed by God by complying with their wish. For, since their inheritance had been promised to them in the land of Canaan, they ought to have been content with that as their dwelling place; nor was Moses permitted to make any alteration in the divine decree. There is also another equally inconsistent matter: in a point of such perplexity, Moses does not, as usual, consult God, but instead gives an immediate answer, which indirectly overthrows God's previous ordinance.

Indeed, their desire was by no means excusable. It would never have entered their minds if they had remembered God's covenant and been satisfied with His goodness. For it is inevitable that the flesh will constantly run riot unless restrained by God's calling.

But God, who knows how to bring light out of darkness, not only pardoned their error but also took occasion to extend His liberality. Thus the land of Bashan and its neighborhood were added to the former boundaries. At the same time, however, He showed, on the other hand, how much better it would have been for them to have been kept together, so that they could have mutually protected each other and lived securely in their appointed dwelling place.

And, after a long period, the Reubenites and Gadites learned from experience that they had been too hasty in wishing for the land they obtained. Nevertheless, through God’s indulgence, what might justly have been harmful to them turned out for their advantage.

We may gather, however, from the result, that Moses was not guilty of any rashness in his interference with God's ordinance. This is clear, first, because he commands that what he now determines is to be ratified and maintained after his death. Second, in the book of Joshua, it is recorded that when the several tribes had their inheritance assigned to them, this country beyond Jordan is excepted, as Moses had granted it to the tribes of Reuben, Gad, and half of Manasseh.

Therefore, it is evident that his decision was approved by God. Moreover, since Moses is often honored there with the title of “servant of God,” we are taught that he did nothing in this matter without God's authority and the guidance of His Spirit. Nor is it by chance that he so often uses God’s name here; rather, he thus implies that whatever he does is suggested by Him.