Two tribes asked Moshe to grant them permission to remain on the east side of the Jordan and not to go into Israel proper. At first Moshe refused, but after they explained that they will settle their families and promised that they will go across the river with the rest of the Israelites to fight the battles to conquer the land and then return, Moshe relented and granted them permission.
Moshe did a strange thing. Only the tribes of GAD and RE’UVEN had asked to remain. When Moshe acceded to their request the Torah says: VAYITEN LAHEM MOSHE LIVNE GAD VELIVNE RE’UVEN VELACHATZI SHEVET MENASHE…, “So Moshe gave to them – to the children of Gad, and the children of Re’uven, and half the tribe of Menashe…” (Num. 32,33) He added half the tribe of Menashe to remain with them when we do not find any request from this tribe to remain and not enter into the Promised Land.
A reason often given was to assure that the tribes remaining and the tribes entering the land would stay in touch with each other. Since it was obvious the two parts of the tribe of Menashe would stay united, they would also guarantee that the other two tribes would do so. Why, however, did he choose the tribe of Menashe?
We might find a clue if we look back to see why Yosef gave Menashe his name when he was born. He called him Menashe because he said: KI NASHANI ELOKIM, “…Hashem has made me forget all my hardship and all my father’s household.” (Gen. 41,51) What usually is thought is that he wanted to forget his father’s home. The contrary is true. He wanted to forget the hardships he encountered there but his intention was that every time he called his son by name he would remember his father’s home.
Hence Moshe had the same intention. This tribe’s presence among the other two tribes would remind them to keep in touch with their brethren tribes.