Hashem says to Avraham in Bereshit 15:18:
בַּיּ֣וֹם הַה֗וּא כָּרַ֧ת יְהֹוָ֛ה אֶת־אַבְרָ֖ם בְּרִ֣ית לֵאמֹ֑ר לְזַרְעֲךָ֗ נָתַ֙תִּי֙ אֶת־הָאָ֣רֶץ הַזֹּ֔את מִנְּהַ֣ר מִצְרַ֔יִם עַד־הַנָּהָ֥ר הַגָּדֹ֖ל נְהַר־פְּרָֽת׃
On that day the LORD made a covenant with Abram, saying, “To your offspring I assign this land, from the river of Egypt to the great river, the river Euphrates:
Whether Nahar Mitzrayim is the Nile or some other river in Egypt, Avraham is promised a massive swath of land, running all the way from Egypt to modern day Iraq.
A similar promise is reiterated in the begining of Yehoshua, 1:4:
מֵהַמִּדְבָּר֩ וְהַלְּבָנ֨וֹן הַזֶּ֜ה וְֽעַד־הַנָּהָ֧ר הַגָּד֣וֹל נְהַר־פְּרָ֗ת כֹּ֚ל אֶ֣רֶץ הַֽחִתִּ֔ים וְעַד־הַיָּ֥ם הַגָּד֖וֹל מְב֣וֹא הַשָּׁ֑מֶשׁ יִֽהְיֶ֖ה גְּבֽוּלְכֶֽם׃
Your territory shall extend from the wilderness and the Lebanon to the Great River, the River Euphrates [on the east]—the whole Hittite country—and up to the Mediterranean Sea on the west.
While this promise does not seem to go as far west as Egypt, it too makes clear our border will go as far east as the Euphrates.
A similar question was asked here:
However, this question merely addresses how to understand the eastern border being the Euphrates. I have 3 additional questions on these pesukim:
- The Pasuk in Yehoshua seems, at least to me, like a more immanent promise, which certainly does not get fulfilled anywhere near the lifetime of Yehoshua. How is this to be understood?
- How do we understand the promise of teritory in Egypt? Why was this promise made to Avraham but later dropped by Yehoshua?
- Assuming we were promised all of this land anyway, why is so much ado made about the request of Reuven, Gad, and Menashe to live on the other size of the Jordan?