Above all else, the First Amendment is a barrier to “prior restraints” - injunctions and licensing requirements aimed at preventing speech from entering the public domain at all.


Just as a new Steven Spielberg film, “The Post,” is celebrating the vindication of that principle in the Pentagon Papers case, President Donald Trump’s lawyers have formally demanded that a publisher cease publication of a new book.


William Blackstone, the English jurist who was a great influence on America’s founding generation, insisted that liberty of the press “consists in laying no previous restraints upon publication.” In his view, free people have the right to put their sentiments “before the public: to forbid this, is to destroy the freedom of the press.”


The founders agreed. In general, they believed that government could not subject speakers and publishers to licensing schemes, or enjoin speech before it ever reached the public.


The Pentagon Papers case, officially called New York Times Co. v. United States, severely tested the limits of this principle. The Washington Post and the New York Times sought to publish material relating to the Vietnam War, leaked by Daniel Ellsberg, that contained secret and classified information.


The government argued that publication should be prohibited. In its view, publishing the papers would put soldiers’ lives at risk, destroy alliances and impair America’s ability to negotiate with its enemies.


Because national security was involved, and because the consequences of publication could have turned out to be horrific, the government’s claims were hardly frivolous. The court ended up siding with the newspapers, but only by a 6-3 margin.


In rejecting the government’s claims, the majority pointed to one principle above all: the heavy presumption against prior restraints. Quoting a 1931 defamation case, Near v. Minnesota, Justice William Brennan emphasized that “the “chief purpose” of the First Amendment is “to prevent previous restraints upon publication.”


In some circumstances, courts do allow damages or subsequent punishment for speech (as in cases of libel) - but outside of the most extreme situations, prior restraints are impermissible.


In this light, turn to the bizarre letter sent this week by Trump’s lawyers to Steve Rubin, president and publisher of Henry Holt and Co. The letter complains that a new book, Michael Wolff’s “Fire and Fury: Inside the Trump White House,” includes material that defames the president and members of his family. With respect to Trump himself, the lawyers object that parts of the book amount to “false light invasion of privacy,” by “arranging otherwise true information in a way to give a false impression.”


The letter threatens to seek “substantial monetary damages and punitive damages.” It adds, “Mr. Trump hereby demands that you immediately cease and desist from any further publication, release or dissemination of the Book, the Article, or any excerpts or summaries of either of them, to any person or entity.”


Despite the unambiguous demand to “cease and desist” from releasing the book, the letter does not say that Trump will go to court to enjoin further publication and dissemination. His lawyers may be aware that after Near v. Minnesota and the Pentagon Papers case, such an effort would inevitably lose.


The book might contain inaccuracies and falsehoods. But what remains astonishing, and much worse than ugly, is that the president of the United States — who took an oath to “preserve, protect and defend the Constitution of the United States” — is threatening a publisher with a costly defamation action, and making a “demand” on him not to publish a book that purports to discuss the inside story of his presidency.


James Madison, the principal author of First Amendment, had something to say about such threats and demands. In Madison’s words, an effort to restrict speech, “more than any other, ought to produce universal alarm.” The reason is that “it is levelled against that right of freely examining public characters and measures, and of free communication among the people thereon.”


In a phrase that should resonate in any self-governing society, and that every president should know, Madison ended with the claim, at once bold and true, that this right “has ever been justly deemed, the only effectual guardian of every other right.”


Cass R. Sunstein is a Bloomberg View columnist. He is the author of “#Republic: Divided Democracy in the Age of Social Media” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.”