In [Bereshit 15:18][1] G-d makes a covenant with Abraham in which He says that He has given certain land to Abraham's descendants.

> בַּיּוֹם הַהוּא כָּרַת יְהֹוָה אֶת אַבְרָם בְּרִית לֵאמֹר לְזַרְעֲךָ נָתַתִּי אֶת הָאָרֶץ הַזֹּאת מִנְּהַר מִצְרַיִם עַד הַנָּהָר הַגָּדֹל נְהַר פְּרָת:     
On that day, the Lord formed a covenant with Abram, saying, "To your seed I have given this land, from the river of Egypt until the great river, the Euphrates river.

At first, one could think that this refers to Eretz Israel after the exile from Egypt. But here G-d described a territory that would be drastically bigger than any jewish land/kingdom there has been.

My interpretation was that this is referring to the times of the Messiah. Based on the facts that: 1) The land of Israel will encompass three new cities when the Messiah arrives. 2) A bad decree can change, while a good decree will never change.*

*I don't have sources for my last two statements. I have either read them on the site or heard them from elsewhere.