And Moses gave inheritance unto the half tribe of Manasseh: and this was the possession of the half tribe of the children of Manasseh by their families. (Joshua 13:29)
As far as can be observed, Manasseh did not formally request this inheritance to the east of Jordan, as did Reuben and Gad (Num. 32:1, 2). Probably it was thought fit to join them with the other two tribes because of the large population of the tribe of Manasseh (Num. 26:34). It is also likely that they had a large number of cattle, as did the other two tribes. The Manassites were good warriors, and perhaps Moses felt it would be well to have them to the east of Jordan as an outer guard, especially the families of Machir and Jair (See Deut. 3:14, 15).