The Reubenites, the Gadites, and the half-tribe of Manasseh crossed over before the Israelites, armed for battle as Moses had instructed them.
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.
The inspired historian seems to be so well pleased with his subject here that he is loth to quit it, and is therefore very particular in his narrative, especially in observing how closely Joshua pursued the orders God gave him, and that he did nothing without divine direction, finishing all that the Lord had commanded him (v. 10), which is also said to be what Moses commanded.
Commenting on Joshua 4:10-19
And the children of Reuben, and the children of Gad, and half the tribe of Manasseh,.... Who were settled on the other side Jordan, in the countries taken from Sihon and Og: passed over armed before the children of Israel; the rest of the tribes: as Moses spake unto them; or ordered them, and as they promised they would, Num 32:17.