Most of Ha'azinu is a song, written in the Torah in 2 parallel columns. Moshe summons Heaven and Earth to stand as eternal witness to what will happen if the Jewish People sin. Israel should be eternally grateful to Hashem, not just for sustaining us in the wilderness, but for bringing us to a land of amazing abundance and defeating our enemies. But, we must be wary, so that this physical bounty does not leads us to become self-satisfied and arrogant.
Physical pleasures can corrupt one's morals.
Hashem will then let other nations subjugate Israel and scatter us across the world. However, the purpose of these nations is as a rod to chastise the Jewish People. When these nations think that it is through their own power that they have dominated Israel, Hashem will remind them that they are no more than a tool of His will. The purpose of the Jewish People is to make mankind aware of the Creator. Hashem will then turn His anger against the enemies of Israel.