With this agreement behind them the two and one-half tribes proceeded to settle in Transjordan until called upon to cross the Jordan River as part of Israel"s fighting force. [source][source][source]
"The eastern territory of Prayer of Manasseh , divided between the clans of Makir and Jair, was essentially identical to the old kingdom of Og. The Makirites took the southern part, that Isaiah , Gilead as far south as the allocation of Reuben and Gad. The Jairites received the region north of Gilead, defined more precisely as Og"s kingdom of Argob. It presumably extended as far north as Mount Hermon and was bordered on the south by the little kingdoms of Maacah and Geshur, just above the Yarmuk ( Deuteronomy 3:13-14). A third entity, Nobah, has no apparent connection with Prayer of Manasseh , but took in Kenath and the surrounding villages ( Numbers 32:42). Kenath (Qanawat) was located about sixty miles due east of the Sea of Galilee, deep in the Hauran Desert." [1][source]
Though these tribes did fulfill their military obligation ( Joshua 4:12-13; Joshua 4:22), their plan was not a good one. It created a potential problem of disunity in Israel. Furthermore it demonstrated a desire for what looked good physically and materially (cf. Lot) in preference to what God said He would give them that they had not yet seen. Distance from the other tribes produced misunderstanding and disunity later ( Joshua 22). The lack of natural defenses on the north, east, and south of Transjordan made this area especially vulnerable to attack by Israel"s enemies. This area was often the first to experience invasion, and Israel lost control of it several times in her later history (e.g, 2 Kings 15:29). [source][source][source]
Whenever the Bible mentions cities of this area allotted to Reuben, especially Heshbon and Mediba, they regularly appear to be under the control of either Moab or Ammon ( Isaiah 15:4; Isaiah 16:8-9; Jeremiah 48:2; Jeremiah 48:45; 1 Chronicles 19:7). Reuben is seldom even mentioned (only in Judges 5:15-16; 2 Kings 10:33; 1 Chronicles 5:6; 1 Chronicles 5:26; 1 Chronicles 12:37; 1 Chronicles 27:16) after the Moabite oppression and never in a way to show that it controlled its allotted land. [2][source]
Transjordan was part of the land God included in the Abrahamic Covenant. However, God"s purpose for Israel at this time was to drive out the Canaanite tribes that lived primarily west of the Jordan. We may assume that if these tribes had simply followed Moses" leadership God would have done something even better for them than what their own plan provided. This is what God often does in such situations, ample testimony to which we find throughout Scripture. [source][source][source]