Why did Moses add part of the tribe of Menashe to join Re'uvean and Gad on the east of the Jordan?

Question

In response to Reuven and Gad's request to settle on the east side of Jordan, Moses includes part of the tribe of Menashe to join them, as it says:

Numbers 32:33:

וַיִּתֵּ֣ן לָהֶ֣ם ׀ מֹשֶׁ֡ה לִבְנֵי־גָד֩ וְלִבְנֵ֨י רְאוּבֵ֜ן וְלַחֲצִ֣י ׀ שֵׁ֣בֶט ׀ מְנַשֶּׁ֣ה בֶן־יוֹסֵ֗ף אֶת־מַמְלֶ֙כֶת֙ סִיחֹן֙ מֶ֣לֶךְ הָֽאֱמֹרִ֔י וְאֶת־מַמְלֶ֔כֶת ע֖וֹג מֶ֣לֶךְ הַבָּשָׁ֑ן הָאָ֗רֶץ לְעָרֶ֙יהָ֙ בִּגְבֻלֹ֔ת עָרֵ֥י הָאָ֖רֶץ סָבִֽיב׃

And Moses gave unto them, even to the children of Gad, and to the children of Reuben, and unto the half-tribe of Manasseh the son of Joseph, the kingdom of Sihon king of the Amorites, and the kingdom of Og king of Bashan, the land, according to the cities thereof with their borders, even the cities of the land round about.

Did any people from the tribe request to be included? If so, why? Or, did Moses force this upon them? Why specifically the tribe of Menashe? How was the division decided (i.e., who should go and who should join the other tribes on the west of Jordan?)

Answer 1

An aish.com article titled "Half of Menashe" offers a number of answers:

The first two tribes, Reuven and Gad, were involved in the negotiations with Moshe from the outset. How did the tribe or part of the tribe of Menashe get themselves involved in this? The text itself is silent; therefore the commentaries feel free to offer different interpretations.

The Ibn Ezra provides a quantitative explanation, theorizing that they did not merit mention until this point because they were so few- even though they were involved in this request from the outset. When one looks at the numbers involved, it becomes clear that the term "half the tribe of Menashe" is imprecise: Of eight families of Menashe, two joined Reuven and Gad in Transjordan while the other six entered the Land of Israel.

Similarly, the Ramban suggests that these two families of Menashe became involved only at this point: When they saw the deal that had been negotiated by the other tribes they jumped on the proverbial bandwagon, hoping to negotiate similar terms. The Ramban explains that numerically they were the smallest group in the tribe of Menashe, and thought that on the other side of the Jordan they would get larger tracts of land. Despite the practical temptation of remaining on the captured side of the Jordan, we cannot help but be surprised when the tribe of Menashe joins the ranks of the "expatriates". After all, they are the children of Yosef, the one leader of a tribe who asked that his remains be returned for burial in the Land of Israel.(9) Furthermore, in last week's parsha, we saw how the women of Menashe, the daughters of Zelaphchad, fought for their right to inherit the Land. It seems strange that the tribe with such a strong bond with the Land of Israel would prefer to remain in trans-Jordan.

Perhaps they never did ask to remain. The Torah does not record such a request; only Moshe's directive is recorded. One could postulate that it was for this precise reason that Moshe requested that they stay behind with the two "secessionist" tribes: Because of their particularly strong love for the Land, perhaps they could serve as a role model for the tribes of Reuven and Gad.

The Netziv suggested that the tribe of Menashe was not interested in staying outside Israel. Rather, Moshe asked them to do so because he knew that the other tribes would need Torah leadership. Menashe's inheritance east of the Jordan was not allotted to them in response to their own request. Moshe initiated the move in an attempt to insure the integrity and continuity of the other two tribes.

Answer 2

Jair Ben Menashe had been responsible for conquering the territory and was given it by Moshe. (In particular the land of Bashan that had been previously ruled by Og). And therefore it was offered to him.

This section of Devarim 3: 12-18 explains how the tribe of Menashe got involved, my source linked here:

http://www.chabad.org/library/bible_cdo/aid/9967/jewish/Chapter-3.htm

12And this land, which we possessed at that time; from Aro'er, which is by the brook of Arnon, and half of Mount Gilead and its cities, I gave to the Reubenites and to the Gadites. יבוְאֶת הָאָרֶץ הַזֹּאת יָרַשְׁנוּ בָּעֵת הַהִוא מֵעֲרֹעֵר אֲשֶׁר עַל נַחַל אַרְנֹן וַחֲצִי הַר הַגִּלְעָד וְעָרָיו נָתַתִּי לָראוּבֵנִי וְלַגָּדִי: 13And the rest of Gilead, and all Bashan, the kingdom of Og, I gave to the half tribe of Manasseh; all the territory of Argob, all of Bashan; that is called the land of Rephaim. יגוְיֶתֶר הַגִּלְעָד וְכָל הַבָּשָׁן מַמְלֶכֶת עוֹג נָתַתִּי לַחֲצִי שֵׁבֶט הַמְנַשֶּׁה כֹּל חֶבֶל הָאַרְגֹּב לְכָל הַבָּשָׁן הַהוּא יִקָּרֵא אֶרֶץ רְפָאִים: 14Jair the son of Manasseh took all the territory of Argob to the boundaries of the Geshurites and the Maachathites, and he called them, even Bashan, after his own name, villages of Jair, to this day. ידיָאִיר בֶּן מְנַשֶּׁה לָקַח אֶת כָּל חֶבֶל אַרְגֹּב עַד גְּבוּל הַגְּשׁוּרִי וְהַמַּעֲכָתִי וַיִּקְרָא אֹתָם עַל שְׁמוֹ אֶת הַבָּשָׁן חַוֹּת יָאִיר עַד הַיּוֹם הַזֶּה: 15And to Machir I gave Gilead. טווּלְמָכִיר נָתַתִּי אֶת הַגִּלְעָד: 16And to the Reubenites and to the Gadites I gave from Gilead to the brook of Arnon, the midst of the brook and the border, until the brook of Jabbok, which is the boundary of the children of Ammon, טזוְלָראוּבֵנִי וְלַגָּדִי נָתַתִּי מִן הַגִּלְעָד וְעַד נַחַל אַרְנֹן תּוֹךְ הַנַּחַל וּגְבֻל וְעַד יַבֹּק הַנַּחַל גְּבוּל בְּנֵי עַמּוֹן: 17the plain, the Jordan and the border thereof, from Kinnereth to the sea of the plain the Sea of Salt, under the waterfalls of Pisgah, eastward. יזוְהָעֲרָבָה וְהַיַּרְדֵּן וּגְבֻל מִכִּנֶּרֶת וְעַד יָם הָעֲרָבָה יָם הַמֶּלַח תַּחַת אַשְׁדֹּת הַפִּסְגָּה מִזְרָחָה: 18And I commanded you at that time saying, "The Lord, your God, has given you this land to possess it; pass over, armed, before your brothers, the children of Israel, all who are warriors. יחוָאֲצַו אֶתְכֶם בָּעֵת הַהִוא לֵאמֹר יְהֹוָה אֱלֹהֵיכֶם נָתַן לָכֶם אֶת הָאָרֶץ הַזֹּאת לְרִשְׁתָּהּ חֲלוּצִים תַּעַבְרוּ לִפְנֵי אֲחֵיכֶם בְּנֵי יִשְׂרָאֵל כָּל בְּנֵי חָיִל: