Thematic Bible: The eastern half assists in the conquest of the country west of the jordan river

And the sons of Reuben and the sons of Gad and half the tribe of Manasseh passed armed before the sons of Israel, as Moses had said unto them. About forty thousand men prepared for war passed over before the LORD unto battle, towards the plains of Jericho.

And I commanded you at that time, saying, The LORD your God has given you this land to possess it; ye shall pass armed before your brethren, the sons of Israel, all that are valiant. Only your wives and your little ones and your livestock (for I know that ye have much livestock) shall abide in your cities which I have given you until the LORD has given rest unto your brethren, as well as unto you, and until they also inherit the land which the LORD your God has given them beyond the Jordan; then shall ye return each man unto his possession, which I have given you.

Also to the Reubenites and to the Gadites and to half the tribe of Manasseh, Joshua spoke, saying, Remember the word which Moses, the slave of the LORD, commanded you, saying, The LORD your God has given you rest and has given you this land. Your wives, your little ones, and your beasts shall remain in the land which Moses gave you on this side of the Jordan; but ye, all the mighty men of valour, shall pass before your brethren armed and help them read more.
until the LORD has given your brethren rest, as he has given you and they also inherit the land which the LORD your God gives them; then ye shall return unto the land of your inheritance and inherit it, which Moses the LORD's slave has given you on this side of the Jordan toward the sunrising.