John Gill 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
Being better disposed towards them, and more satisfied with the reasonableness of their request, it being explained to him:

if you will do this thing ;
which they had promised:

if you will go armed before the Lord to war ;
they had said they would go ready armed before the children of Israel, but Moses expresses it "before the Lord"; which is more agreeable to their encampment and order in marching, for not the standard of Reuben but that of Judah went foremost, yet the standard of Reuben marched directly before the sanctuary borne by the Kohathites, (Numbers 10:18Numbers 10:21) , and so might be properly said to go before the Lord, who dwelt there.