“And the people of Gad and the people of Reuben answered, ‘What the Lord has said to your servants, we will do. We will pass over armed before the Lord into the land of Canaan, and the possession of our inheritance shall remain with us beyond the Jordan'” (Numbers 32:31-32, ESV).
In Numbers 32 the Israelites are preparing for the conquest of the promised land. The people of the tribes of Reuben and Gad requested that Moses allow them to settle in lands on the east side of the Jordan River, which was not within the boundaries of the promised land. They wanted to settle east of the Jordan because they owned large herds of livestock and there was good grazing land there.
At first Moses resisted their request pointing out that they were disobeying God just like their fathers who died during their forty-year wandering in the wilderness because they rebelled against the plan to possess the land God had promised.
So, the tribes of Reuben and Gad struck a deal with Moses that they would settle their families and livestock on the east side of the Jordan but their fighting men would take up arms and lead in the battles against the inhabitants of the land until all the people groups in the land of Canaan were subdued. If they stayed until the war in Canaan was over, then the lands on the east side of the Jordan would be their inheritance.
Steve's Bible Meditations 