A historic agreement establishes a clear standard for claiming a homeland, tying the right to a territory directly to active participation in the national struggle. The leadership of certain tribes receives a promise for their own land, but this right hinges entirely on their willingness to march to the front lines alongside the rest of the nation, before God.
A natural question arises regarding the actual transfer of this territory. Why are Joshua and the national leaders commanded to grant the land to these tribes in the future, if Moses already appears to have given it to them? The primary approach among commentators is that, at this initial stage, Moses has not yet granted absolute ownership over the entirety of the conquered territories. He merely permits them to take control of a few specific cities in the region of Gilead to build fortresses for their families and flocks, while the rest of the land remains desolate.
The final, permanent transfer of the entire region depends entirely on keeping their promise. Only after these tribes cross the Jordan River and fight will the territory be granted to them as an eternal inheritance. Therefore, the actual transfer of ownership can only be completed in the future by Joshua [רמב״ן, אור החיים, הטור הארוך]. The instruction directed at the future leadership simply outlines the practical result of the condition being successfully met [אבן עזרא].
Beyond the practical instructions, this arrangement carries a profound message of a shared destiny. The success of the tribes settling within the land of Israel is deeply connected to the success of those settling across the Jordan River. If the pioneer forces go out to battle, the entire nation of Israel will achieve victory on both fronts simultaneously [אור החיים].
Conversely, a strict consequence awaits if the tribes of Gad and Reuben fail to honor their commitment and refuse to cross the river willingly. In such a scenario, they will not merely lose their claim to the territory across the Jordan. Their families will be evicted from the cities they have already begun to settle. They will then be brought by force across the river into Canaan, where they will be compelled to accept a standard inheritance alongside the rest of the tribes [רמב״ן, הטור הארוך].