Almost all of Ha'azinu is a song, written in the Torah in two parallel columns. Moshe summons the heavens and the earth to stand as eternal witnesses to what will happen if the Jewish People sin and do not obey the Torah. Hashem's kindness is such that Israel should be eternally grateful, not just for sustaining them in the wilderness, but for bringing them to a land of amazing abundance, and for defeating their enemies. But, this physical bounty leads the people to become and over-indulged.
Physical pleasures corrupt the morals of the people.
They worship empty idols and powerless gods, and indulge in all kinds of depravity. Hashem will then let nations with no moral worth subjugate Israel and scatter them across the world. However, their only purpose 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 that a tool to do His will. The purpose of the Jewish People is fundamental - that man should know his Creator.