Logan Act is too vague to prosecute Michael Flynn. Or anyone

The resignation of National Security Adviser Michael Flynn grew out of Department of Justice concerns that he had violated the Logan Act, a law from 1799(!) that bars private citizens from engaging in international diplomacy. The law as written applied to Flynn even though he was working for the president-elect when he engaged in a phone call with Russian ambassador to the U.S.

But there's a more serious problem, which should be kept in mind in case there's an investigation of whether Donald Trump violated the law: It is probably unconstitutional. Enacted by the Congress that brought you the Alien and Sedition acts, the law is too vague for enforcement. And it violates free-speech standards that are the law today but went unrecognized by the John Adams administration.

Ordinarily I would start a column like this by reviewing the binding judicial interpretations of the law over the centuries. The problem is, there aren't any. Exactly one person has been indicted for violating the law, a Kentucky farmer named Francis Flournoy who in 1803 wrote a newspaper article advocating the creation of a separate country that would ally itself with France. Flournoy left Kentucky before the trial could start. It appears, based on research by my late colleague Detlev Vagts, that no one has been prosecuted under the law since.

The text of the law says that a citizen is guilty of a crime if without authority he "directly or indirectly . . . carries on any correspondence or intercourse with any foreign government . . . with intent to influence [it] . . . in relation to any disputes or controversies with the U.S. or to defeat the measures of the U.S." The punishment is an unspecified fine and up to three years in prison.

The law doesn't define "disputes or controversies" between the foreign country and the U.S., nor does it say what it means to "defeat the measures of the U.S."

That's enough to make the statute unconstitutionally vague. What if I am simply speaking as a private citizen to the prime minister of a foreign country about its relations with the U.S. and those relations sometimes involve disagreement? Am I committing a crime, even if I'm not purporting to do diplomacy?

And what would it mean for me to be trying to "defeat" U.S. measures? Does that include trying to convince the foreign government that it shouldn't agree with U.S. policy?

If a criminal law doesn't clearly specify what actions would be criminal, it's unconstitutional to apply it.

The uncertainty about whether Flynn's act would have violated the law is a case in point: the very vagueness of the law would make its application unconstitutional.

In 1964, a federal district court adjudicating a private lawsuit that included an allegation of violation of the Logan Act "invited" Congress to amend the law "by using more precise words than 'defeat' and 'measures.'" The invitation reflected the intuition that the law was too vague to be applied fairly.

The court noted that the law is still on the books even though it's very old. It quoted Shakespeare's "Measure for Measure": "The law hath not been dead, though it hath slept."

It's true that antiquity alone is no proof that a law is invalid. But changing times can give rise to serious constitutional problems -- for example, the changed meaning of the First Amendment.

The Federalist Congress that passed the Logan Act in 1799 had also passed the Sedition Act of 1798. That law made it a crime to impede any act of government or prevent a government officer from performing his duty. It was applied by Federalist judges to jail and fine Republican newspaper editors.

Today the Sedition Act is considered the very model of a law that violates of the First Amendment's guarantee of freedom of the press. At the time, Federalist judges, including Supreme Court Justice James Iredell, argued that the First Amendment only prohibited "prior restraint" on the press, which is to say pre-publication censorship. Once an article was published, Iredell maintained, members of the press could be held criminally liable for what they had written.

The Logan Act would almost certainly violate today's understanding of the First Amendment. It's a direct prohibition on speech, so would have to satisfy strict scrutiny, the highest level of judicial review.

That means the law would have to serve a compelling government interest and adopt the least restrictive means to achieving that goal.

There may arguably be a compelling interest in private individuals not doing diplomacy, for example if someone claimed to be representing the government. But can there really be a compelling government interest in American citizens not speaking to foreign governments about matters connected to U.S. policy, which could include anything from oil fields to intellectual property? That seems unlikely. It also seems far from certain that there is a compelling interest in American citizens being banned from seeking to undercut American diplomatic initiatives abroad. Lots of non-Americans do that already.

As for the least restrictive means, surely that could be accomplished by a law that simply prohibited Americans from pretending to speak on behalf of the U.S. Or the government could try to prohibit efforts to subvert U.S. foreign policy that actually have a chance of succeeding.

It emerges that the Logan Act is, constitutionally speaking, a "paper tiger," as Vagts once called it. We should keep that in mind as conversations continue about potential Trump administration violations. It's a bad idea for private citizens to do official diplomacy. But making it a crime would require a new statute, one carefully crafted to meet the constitutional standards of 2017, not 1799.

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Noah Feldman, a Bloomberg View columnist, is a professor of constitutional and international law at Harvard University and the author of six books, most recently "Cool War: The Future of Global Competition."