וַיֹּ֣אמֶר יְהוּדָה֩ לְשִׁמְע֨וֹן אָחִ֜יו עֲלֵ֧ה אִתִּ֣י בְגוֹרָלִ֗י וְנִֽלָּחֲמָה֙ בַּֽכְּנַעֲנִ֔י וְהָלַכְתִּ֧י גַם־אֲנִ֛י אִתְּךָ֖ בְּגוֹרָלֶ֑ךָ וַיֵּ֥לֶךְ אִתּ֖וֹ שִׁמְעֽוֹן׃
И сказал Иуда Симеону, брату своему: 'Пойдем со мной на мою судьбу, чтобы мы могли сражаться против хананеев; и я тоже пойду с тобою на судьбу твою.' Итак, Симеон пошел с ним.
Chizkuni
שמע ה׳ קול יהודה, “hear o Lord the voice of Yehudah!” Whenever Yehudah finds himself in difficulties and appeals to You for help on behalf of his people, grant his plea! He is singled out for this as he normally leads the rest of the tribes in warfare. (Compare Judges 1,2) The same applied to Reuven, as he had been the first tribe to cross the Jordan and face the enemy. Compare Joshua 6,7.) Shimon did not need to be mentioned separately as that tribe always made common cause with Yehudah as reported in the Book of Judges. [His territory was an enclave within the territory of Yehudah. Ed.] (Compare Judges 1,3) A different interpretation of Moses’s pleading: יחי ראובן, “may Reuven live!” Seeing that the founding father of the tribe of Reuven had committed a serious offense against his father Yaakov, and the commandment of honouring father and mother is accompanied by the promise that people doing so would enjoy long life, (Exodus 20,12) it could be inferred that people failing to do so would have their lives cut short, Moses blesses the tribe, emphasising that the descendants of Reuven should not be affected by something their founding father had done wrong.
Rashi
but his brothers did not have many sons and all their family did not multiply as much as the children of Judah They did not have as many children as the children of Judah. Since they were few, and Judah’s territory was too large for them, Judah accepted Simeon in its territory, because Simeon had no share in the land of Israel, as it says (Gen. 49:7): “I shall divide them in Jacob and scatter them in Israel,” and we learned in Genesis Rabbah (Shittah Hadashah): “I shall divide them among the Children of Israel to station scribes in Israel in the synagogues, etc.” Now how do we know that they agreed [to give them a share of their territory]? For it is written (Jos. 19:9): “Out of the lot of the children of Judah was the inheritance of the children of Simeon, for the portion of the children of Judah was too large for them; therefore the children of Simeon inherited in the midst of their inheritance,” lest the beasts of the field outnumber them. Now as regards the verse (ibid. v. 1): “And the second lot came out to Simeon,” this is what happened: They cast lots to choose which tribe would accept Simeon in its territory, and the lot fell upon the territory of Judah. And the meaning of the verse (Jud. 1:3): “And Judah said to Simeon his brother: Come up with me into my lot, and we will fight against the Canaanites, and I will also go with you into your lot,” refers to “the lot I have given you.”