We will cross over before the Lord into Canaan armed, but the property we inherit will be on this side of the Jordan.

Numbers 32:20-42
He said to them, “If the Gadites and Reubenites, every man armed for battle, cross over the Jordan with you before the Lord, then when the land is subdued before you, you must give them the land of Gilead as their possession.  But if they do not cross over with you armed, they must accept their possession with you in Canaan.”
The Gadites and Reubenites answered, “Your servants will do what the Lord has said.  We will cross over before the Lord into Canaan armed,but the property we inherit will be on this side of the Jordan.”
The descendants of Makir son of Manasseh went to Gilead, captured it and drove out the Amorites who were there.  So Moses gave Gilead to the Makirites, the descendants of Manasseh, and they settled there.
So they work out a practical plan to enable the fighting force to leave the livestock and their dependents behind, safe and secure. Part of the tribe of Manasseh conquers Gilead, and Moses gives them this land for their own.
Why?
Describe the 2.5 tribes settled in east of Jordan river. For they found east part of Jordan is good place for living and God subdue their enemy.
They agreed cross over the Jordan with arm and fight with his brothers, so Moses agreed their request as Lord commanded.
How?
New generation started build their promised land as their home country. But there are many enemy and battle before them. When people are settle down, we are easy lose our faith to the Lord.