Rashi on the Former Prophets

Furthermore, when Yehoshua sent them off. He sent off the half-tribe of Menashe in the same manner as he sent off the Bnei Reuvein and the Bnei Gad.

[Divide the spoils] with your brothers— Referring to Bnei Gad and Bnei Reuvein. There are some who interpret “brothers” in referrence to those who remained behind to guard the cities and the women and children. [Even though] they did not cross the Jordan— with the armed division, nevertheless they received a share of the spoils.

To go up against them to war. [They were prepared to do this] because בָּמוֹת were prohibited or the Beis ha Mikdosh. These בָּמוֹת were permitted during the period before the establishment of the Mishkan, but were prohibited so long as the Mishkan was intact. They were considered a threat to the unity of the Klal Yisroel and to the centrality of the Mishkan. once the Mishkan was established at Shiloh.

If the land of Your posession is impure. Since the Holy blessed is He, did not choose to rest His Presence there.
Do not rebel against us.—וְאוֹתָנוּ אַל תִּמְרדוּ This is the same as—וּבָּנוּ אַל תִּמְרדוּ

Almighty God is Adonoy. He is God over all divine beings. He is Adonoy, Who knows that our intent was not rebellion etc. By repeating this declaration twice, [they declared] that He is God in this world and in the World to Come.
Do not save us this day. They said this to His Divine Presence.

Let Him prosecute—Let him punish us.

Our anxiety of this. Because of anxiety, and the fear of insulting words, have we done this, as is further explained in the Scripture: ‘Lest tomorrow your children will say,’ Lest they insult our children— when they go to bring sacrifices in the Mishkan at Shiloh— lest they say to them, ‘What have you to do with Adonoy?’ Did he not set a boundary between us and you, namely, the Jordan. This was our fear and thus we made it. Every word, דְּאָגָה, found in Scripture expresses fear, similar to, “I am concerned [fear] that the Jews etc.” [These were the words] of Tzidkiahu.

Rather it will bear witness that we have not withdrawn ourselves from the law governing the altar.

And they the Bnei Yisroel blessed God. The Bnei Yisroel thanked Adonoy.

And the Bnei Reuvein and Bnei Gad called the altar, for it is a witness. This is one of the shortened verses. One more word should have been added: And the Bnei Reuven and Bnei Gad called the altar ‘witness,’ [for it is etc.]