When in Biblical times, the Land of Israel was settled by the Israelites, two and a half of the twelve tribes chose to live on the Eastern side of the Jordan river: Reuven, Shimon and half of Menasheh. They felt the land there would be better for their animals.
In our Parasha we see them pitching their proposal to Moses, who initially thought that they were shirking their responsibilities towards the rest of the tribes by not helping them conquer the land. Their proposal was to take the lead in conquering the land, and only upon the war’s completion return to the Eastern side of the Jordan to live with their families. Moses accepts their proposal.
An interesting change in word order is noticed when reading the 2.5 tribes’ proposal and Moses’s response. Commentators view in this a lesson about proper priorities. The tribes say, “We will build enclosures for our sheep here, and cities for our children” (Num 32:16) Moses responds, “Now build yourselves cities for your children and folds for your sheep…” (v. 24). The tribes seem to pick up on his subtlety and respond, “We will do as you have ordered. Our children, wives, property and livestock will remain here…” (vv. 25-26).
Moses was teaching them what we all know: that family should always come before business.