On the eve of the Israelites' passage into the Promised Land, Moses teaches them a song that marks the moment and that they themselves are to sing now and throughout the generations. It's only the second such song in the Torah—the first being the rousing tribute to God after the parting of the Red Sea back in Exodus: "I will sing to the Lord, for He has greatly exceeded." 

This time, the song could not be more different. Although it opens with predictable praise of God ("He is the Rock, His work is perfect / for all His ways are justice"), the subject quickly shifts to Israel's failure to keep the faith. "They are a perverse and crooked generation," we read, "Do you thus requite the Lord / O foolish people and unwise?"

It gets much worse. After recalling the great deeds God did for Israel, we read that Israel "grew fat, and kicked. . . . They provoked Him to jealousy with strange gods. . . . Of the Rock that begot you, you are unmindful / and have forgotten God who formed you." God's rage is captured in blinding colors. "And He said, I will hide My face from them / I will see what their end will be. . . . For a fire is kindled in My anger / and shall burn to the nethermost parts of the earth. . . . I will heap mischiefs upon them / I will spend My arrows on them / They shall be sucked empty by hunger, and devoured with burning heat / and with bitter destruction. . . . For the day of their calamity is at hand. . . . I will make My arrows drunk with blood / and My sword shall devour flesh."

What is going on?

Understanding this sour song begins with the broader context of the last few chapters of Deuteronomy. Indeed, this is merely the latest in a string of similarly dour passages: the horrible destruction, detailed in Deuteronomy 28, that will result from Israel's failure to lead a moral life; the ceremony on the mountains in which curses are hurled on those who violate the moral code; God's telling Moses that after conquering the land, the Israelites will fail and face His awful wrath; and more.

Clearly, then, the song is part of a larger effort by Moses to convince the Israelites that they should be thinking less about the glories of their imminent conquests and more about the terrible possibilities that can result from their sins. But why now?

Every sentiment has its moment. If the previous generation's greatest sin was a failure of spirit, the lingering slave-mentality that filled them with terror at the prospect of any military confrontation, the current generation of Israelites faces a completely different threat: hubris. Fresh from their military successes, roused and ready to fight their way into their old-new land, they do not see, as Moses does, the powerful danger lurking: that their victories will go to their head, leading them to moral dissolution and causing them to lose sight of the very purpose for which they were brought out of Egypt.

The song, in other words, is tailored to the needs of a generation of Israelites primed for battle, ready to conquer the Promised Land. Yet the lessons it contains are no less apt for our own time.

Implicit in the song, and in the passages surrounding it, is a pernicious pattern of human endeavor. Failure leads to remorse and repentance, which leads to fresh resolve and success; success leads to arrogance and to forgetting how we grew strong and prosperous—which in turn leads to failure. Every human society—indeed, every individual—is prone to this cycle.

Can it be broken? It can be, we learn, if we maintain an everlasting respect for how bad things can be, a gratitude for everything we have received, and a recognition of the simple fact that it is all up to us. Just as God told Cain way back in Genesis: If we do good, that is fine. If not, sin crouches at the door. And still we may rule over it.

David Hazony is author of The Ten Commandments: How Our Most Ancient Moral Text Can Renew Modern Life, published this week by Scribner.