G-d then tells Moshe and Yehoshua that after entering the Land, the People will be unfaithful to Him, and begin to worship other gods. Moshe records the song in writing and teaches it to The People of Israel. Most of Parshat Ha'azinu is a song, written in the Torah in two parallel columns. Moshe summons heaven and earth to stand as eternal witness to what will happen if the Jewish People sin. G-d'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. G-d 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, G-d will remind them that they are no more than a tool to do His will. The Jewish People's purpose is to make mankind aware of the Creator.