Moses’ initial reaction to Reuben and Gad’s request to stay behind is one of shock: Would they really leave their brothers to conquer Canaan while they enjoyed a territory away from the Promised Land?
After hearing the real reason for Reuben and Gad’s request—that they preferred the territory east of the Jordan for their inheritance—and affirming that the tribes would follow the rest of Israel into Canaan, Moses agreed to let them stay.
With a condition. Moses told them that “if they will arm [themselves] before the Lord for the war, and all of [their] armed men cross over the Jordan before the Lord” until the war is over, they’re free to take the land in the Transjordan. It’s as simple as that. If they refuse, it’s a sin, and God will hold them accountable.
But that phrase “before the Lord” has a deeper meaning than appears at first. For instance, the Hebrew phrasing in Numbers 32:17 has Reuben and Gad offering to become “shock troops,” which is a special military group that usually attacks first to disrupt enemy lines.
If you go forward to Numbers 32:20, Moses agrees with their proposal. And when you go even more forward to Joshua 6:9, during the assault on Jericho, the Text says that the Ark went before the main army, but the “armed men” went before the priests with the trumpets (which came before the Ark).
Traditionally, that group of “armed men” is thought to be the tribes of Reuben and Gad. But we don’t really need to lean on tradition here, since Joshua 4:12-13 explicitly states that the two-and-a-half tribes of Reuben, Gad, and the half tribe of Manasseh crossed the Jordan before Israel — 40,000 armed men equipped for war.
The total population for those tribes equals 110,580 (Numbers 26:7, 18, 34), so the number of elite fighters chosen to assist the other tribes in the conquest of Canaan represents about one-third of their able-bodied men. The rest would stay behind to fortify the cities in the Transjordan and rebuild the cities taken from the Midianites.
You can see the advantage in having a highly-mobile, highly-trained group of warriors to lead the charge into Canaan, and yet, for these men, it was an enormous sacrifice. Whereas most of the tribes would be allowed to start their lives in their new homes, they spent another seven years fighting for land they didn’t get to keep.
It’s not until Joshua 22 when they’re dismissed from active service and allowed to return to their homes. Joshua tells them they’ve fulfilled their duty and may “return to their tents.” As with everyone else, they should be careful to maintain the laws of God.