The E. Jordan tribes had promised to help the others to conquer the territory W. of the Jordan (Numbers 32:28-32 *).
Related Commentaries of Deuteronomy 3:18
Deuteronomy 3:18-20
18 And I commanded you at that time, saying, The LORD your God hath given you this land to possess it: ye shall pass over armed before your brethren the children of Israel, all that are meetb for the war.
19 But your wives, and your little ones, and your cattle, (for I know that ye have much cattle,) shall abide in your cities which I have given you;
20 Until the LORD have given rest unto your brethren, as well as unto you, and until they also possess the land which the LORD your God hath given them beyond Jordan: and then shall ye return every man unto his possession, which I have given you.