Jamieson, Fausset and Brown on Joshua 4:12-13

12-13. the children of Reuben ... passed over armed before the children of Israel--There is no precedency to the other tribes indicated here; for there is no reason to suppose that the usual order of march was departed from; but these are honorably mentioned to show that, in pursuance of their promise (Jos 1:16-18), they had sent a complement of fighting men to accompany their brethren in the war of invasion.