When the land to the east of the Jordan River had been allocated, Moses gave a special command for the tribes who were to settle there: since the Lord had given the land to them and the other tribes still had to take possession of their lands, these eastern settlers were to participate in the remaining conquest, and help their western brethren.
18 “And I commanded you at that time, saying, ‘The LORD your God has given you this land to possess. All your men of valor shall cross over armed before your brothers, the people of Israel.