Rashi: When He gave to them "Nachalas Goyim", He made them know His strength and Gevurah.

Rashi citing Midrash Tanchuma (Bereishis 11): He wrote [in the Torah] for Yisrael Ma'ase Bereishis, to inform them that the land is His, and He may settle in it whom He wants. The nations cannot tell Yisrael 'you are robbers - you conquered the land of the seven nations.'

Radak: Even though in Egypt, He showed to them Nifle'osav and His great wonders, His strength He showed only later, when He dispossessed seven great awesome nations from their land, and made Yisrael dwell in it. And so Moshe said - "Pen Yomeru ha'Aretz Asher Hotzeisanu mi'Sham mi'Beli Yecholes Hashem..." (Devarim 9:28). They will say, even though He had strength to take them out of their servitude to us, He did not have strength to expel the [Kena'ani] nations from their land, and give it to [Yisrael]. It says Higid, for He told them [that He will expel the nations] before He did so, lest people say that it was chance.