The poem of Haazinu begins in Devarim 32:1. But the introduction to the poem begins in the previous chapter:
“The LORD said to Moses: You are soon to lie with your fathers. This people will thereupon go astray after the alien gods in their midst, in the land that they are about to enter; they will forsake Me and break My covenant that I made with them. […] Yet I will keep My countenance hidden on that day, because of all the evil they have done in turning to other gods. Therefore, write down this poem and teach it to the people of Israel; put it in their mouths, in order that this poem may be My witness against the people of Israel.” (Devarim 31:16-19)
Why did the Sages not place the introduction and the poem in the same parasha?
I think the answer can be found by paying attention to the history of Israel as described in Haazinu:
“When the Most High gave nations their homes, and set the divisions of man … His own nation remained God’s portion .. He found [Israel] in a desert region” (Devarim 32:8-10)
No mention is made at all of the Avot, their dedication to God, and God’s promise to them. It is as if the history of Israel began in the desert.
This stands in significant contrast to the rest of the book of Devarim. There are frequent mentions of the Avot, and how they are the reason for God choosing Israel:
“And because He loved your fathers, He chose their heirs after them” (Devarim 4:37)
“Yet it was to your fathers that the LORD was drawn in His love for them, so that He chose you, their lineal descendants” (Devarim 10:15)
And because of God’s love of the Avot, He promised Israel he will always protect them. Right before teaching the poem, Moshe reminds Yehoshua of that commitment:
“Then Moses called Joshua and said to him in the sight of all Israel: ‘Be strong and resolute, for it is you who shall go with this people into the land that the LORD swore to their fathers to give them, and it is you who shall apportion it to them. And the LORD Himself will go before you. He will be with you; He will not fail you or forsake you. Fear not and be not dismayed!'” (Devarim 31:7-8)
The theme of God’s infallible commitment to the Avot was essential for the nation at this point in their history. They were about to begin the conquest of the land, and would enter battle against their foes. In the previous generation, when the Spies returned with a frightening report, the people lost their courage and were unwilling to enter the land. So now Moshe needed to assure them that God would not forsake them and would guarantee their success in the campaign.
But while those assurances were necessary for the conquest, they could prove perilous once the nation had settled in the land. They might feel that their success was guaranteed no matter what they do. They would even go so far as to worship other gods, despite God being the one who delivered them the victories in the first place.
So to mitigate against that arrogance, God told Moshe to teach the people the poem of Haazinu. In this song there would be no mention of the merit of the Avot, or any other reason for God’s choice of Israel. Even Israel’s covenant with God at Sinai is absent. All that is left is God saving Israel in the desert.
With this backdrop, the ungratefulness and unfaithfulness of Israel is starker than anywhere else in the Torah. Even the introduction to the poem mentions the Avot (“When I bring them into the land flowing with milk and honey that I promised on oath to their fathers” – Devarim 31:20). By beginning the parasha directly with the poem, the reader is expected to start fresh, and consider this different perspective. Hopefully, the difficult picture described in the song will enter the hearts of the listener, who can therefore try to avoid its fate.