By Cass R. Sunstein

Bloomberg Opinion

What pushed former Defense Secretary James Mattis over the edge, to denounce President Donald Trump, in the strongest possible terms?

Only the former general knows for sure, but a clue is provided by the title of his statement: “In Union There Is Strength.”

Another clue is provided by the most important words in his text: “Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try. Instead he tries to divide us.”

With those words, Mattis is signaling a national challenge that goes back to the founding era, that almost derailed the American project from the very start, that helped start the Civil War and that has had to be managed with great care during every national crisis.

Shortly after the American Revolution, the new nation was at grave risk of falling apart. To many people, diverse affiliations and identities made it difficult to speak of the “United States of America.” Under the Articles of Confederation, intense loyalty to states, and competition among states, seemed to outstrip loyalty to the nation. Prominent politicians fueled the divisions.

The Constitution was designed to solve that problem. You can see what its framers had in mind if you look an early draft of the document.

It began: “We the People of the States of New Hampshire, Massachusetts, Rhode Island and Providence Plantations, Connecticut, New York,” and so forth, “do ordain, declare, and establish the following Constitution.”

The final version has a radically different start: “We the People of the United States, in Order to form a more perfect Union .”

The early draft suggests that the Constitution is created by the states; it sees “We the People” as citizens of their states, first and foremost. The final text emphasizes national citizenship. And rather than going directly from “We the People” to the act of establishing the Constitution, it declares the purposes of that act — and gives pride of place to the formation of “a more perfect Union.”

That project was designed to overcome disparate allegiances, interests and ideologies, producing “factions,” which James Madison regarded as an omnipresent threat. More specifically, the institution of slavery was a moral as well as practical threat to the existence of that more perfect union — and of course its legacy is at the heart of some of our divisions today.

Mattis’ concrete concern is what he sees as the misuse of the military to maintain public order. As he understands it, that task “rests with civilian state and local leaders who best understand their communities and are answerable to them.” In his view, our response to protests should not be militarized.

The power of Mattis’ text lies in linking that claim with the broader idea of national unity. If the military is deployed too readily, we will see “a conflict — a false conflict — between the military and civilian society.” That is dangerous; it is what we see in authoritarian societies.

Mattis views a conflict between the military and civilian society, concocted during a series of protests over racial injustice, as distinctly threatening to national unity. There is a reason that, by tradition, the military is nonpartisan. Whether generals or captains or privates, soldiers protect the American people, not a party or a politician.

Mattis refers explicitly to the Constitution. But Madison himself was deeply concerned about the potential weakness of “parchment barriers,” used to protect “against the encroaching spirit of power.” In 1788, Madison asked: “Is there no virtue among us? If there be not, we are in a wretched situation. No theoretical checks - no form of government can render us secure.”

The very project of self-government depends on a shared understanding that, for all of our divisions, Americans are engaged in a common enterprise — and that national leaders are committed, above everything else (including their own self-interest), to that enterprise. When the president does not share that commitment, we are in a wretched situation.

Mattis is aiming to get us out of it. Who will join him?

Cass R. Sunstein is a Bloomberg Opinion columnist.