Moshe summons heaven and earth to stand as eternal witness to what will happen if the Jewish People sin. 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.
Physical pleasures corrupt their morals.
They worship empty idols and indulge in depravity. Hashem will then let nations with no moral worth subjugate Israel and scatter them 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 to do His will. The purpose of the Jewish People is to make mankind aware of the Creator.