Deuteronomy 30:5
And the LORD thy God will bring thee into the land which thy fathers possessed, and thou shalt possess it; and he will do thee good, and multiply thee above thy fathers. And the LORD thy God will bring thee into the land which thy fathers possessed, and thou shalt possess it; and he will do thee good, and multiply thee above thy fathers.
The scattering addressed by the promise came because Israel failed to be God's missionaries and built walls of division between themselves and the peoples around them, after which they were dispersed among other nations. God scattered them in this way so that the knowledge of His truth might be carried to the world.
The restoration to the land is conditional, promised only when God's scattered people return to Him in penitence and faith and keep His commandments. It is for those who are loyal and true and submissive, and for those obedient to His commandments. When they so return, God will gather His scattered people together again. He will bring them back into the land which their fathers possessed, and they shall possess it. There He will do them good and multiply them, even above their fathers, and again show them His favor.
Moses foresaw this return to the fathers' land, and saw it followed by Israel coming under Roman rule. Its fulfillment shows that His word remains faithful through the centuries.