Where did the half-tribe of Menashe come from all of a sudden?
Initially, Moses was approached by the tribes of Gad and Reuben, who wanted to remain in Trans-Jordan where there was ample grazing land (Numbers 32:1-2). Moses explains that only after they participate in the conquest of the Land with their tribal brothers would they be permitted to receive Trans-Jordan as their inheritance; they agree.
And then, when Trans-Jordan is given, we suddenly find half of the tribe of Menashe entering as partners. When and why did Menashe enter the scene?
The Ramban (Nahmanides) suggests that although Moses had initially been approached only by Gad and Reuben, it soon became apparent that the land in Trans-Jordan was plentiful enough to accommodate another partner. Moses called for volunteers, and members of Menashe responded, "perhaps because they were also herdsmen seeking grazing lands" (Numbers 32:33, Ramban ad loc.).
I would add that perhaps they volunteered for another reason: Perhaps they were opportunists, seeking lush farmland and desiring to be distanced from the more spiritual tribe of Judah, the more centralized location of the Sanctuary and the divinely centered capital of Jerusalem. In this regard, the people of Menashe were true to their tribal forebear: Remember that Joseph says about the name he chose for his eldest son, Menashe, "God has enabled me to forget (nashe, forget) all my toil and everything involved in my father's house."
Apparently, this included a spiritual amnesia - a "forgetting" of many of the Abrahamic traditions. And Menashe was the politically adept, linguistically fluent son who aided his father in selling grain to representatives of various countries; he was not like his younger brother Ephraim, who studied Torah with his elderly grandfather Jacob. So it would make sense that this eldest of Joseph's sons caused the family to move closer to Egypt-idolatry and further from Israel-Judaism. And indeed, it would seem that these two-and-a-half tribes did attempt to build an altar to idolatry in Trans-Jordan during the period of Joshua, until they were dissuaded by a delegation composed of Phineas and representatives from the rest of the tribes (Joshua 22:12-19).
Apparently, geographical distance from Jerusalem creates ideological difference as well - until this very day.
A very different and even opposite scenario is suggested by the Netziv, Rabbi Naftali Zvi Yehuda Berlin, in his late 19th-century biblical commentary called Ha'amek Davar. He insists that Moses specifically chose half of Menashe to join Gad and Reuben in Trans-Jordan in order to upgrade Reuben's spiritual standards. Moses was concerned lest "far from the eye makes one far from the heart." After all, the enemies of the land of Israel and the Torah of Israel were Dathan and Abiram, scions of the disgruntled and "disinherited" tribe of Reuben; letting the tribe settle so far from mainstream Israel was certainly asking for trouble.
The people of Menashe, on the other hand, were perfect "religious supervisors" for the less trustworthy Reuben. Did not the wise, righteous, committed lovers of Israel, the daughters of Zelophehad, come from Menashe? And was not Jair (literally "he will shine forth light,"), the son of Menashe (Deut. 3:14), a great Torah scholar considered by our talmudic sages to have been equal to the majority of the Sanhedrin?
Hence, the sincerely Zionistic and learned tribe of Menashe would be the perfect individuals to religiously influence the suspect tribe of Reuben far from the spiritual center of the land. The Reubenites were to serve in a capacity similar to Chabad emissaries or Amiel rabbis of the Joseph Straus Rabbinical Seminary - emissaries to Jews in far-flung places who bring the traditional religious message to those who are distanced from it, geographically as well as ideologically.
And why only half the tribe of Menashe? When someone is sent to a far-flung community, hopefully he will influence them - but the danger always exists that they will influence him. If the emissaries have uncles, aunts, brothers, sisters and cousins closer to the religious center, chances are that there will be frequent communication. This would increase the likelihood that the emissaries will remain firm in their commitment to inspire their neighbors in Trans-Jordan.
The writer is the founder and chancellor of Ohr Torah Stone Colleges and Graduate Programs, and chief rabbi of Efrat.