Donald Trump revived a century-old legal debate Tuesday morning when he tweeted that anyone who burns the United States flag should face “loss of citizenship” or a year in jail.

The Supreme Court held in 1989 that flag burning is a protected exercise of free speech, striking down 48 states’ laws prohibiting the act.

Trump’s casual mention of stripping people of their U.S. citizenship as punishment, however, has an older and more tangled legal history — one that critics fear may get a fresh look under his presidency.

Today, U.S. citizens cannot be stripped of their citizenship unless they renounce it or were found to have lied on their naturalization forms. Citizenship is seen as such a sovereign right that the U.S. government can execute a convicted traitor but cannot denationalize him. (“They can kill you, they can execute you, they can make it a capital offense, but they can’t take away your citizenship,” legal scholar Jack Chin told Yahoo News.)

But it wasn’t always that way. During much of the 20th century, citizenship was considered a conditional right that could be revoked by the government, particularly for “naturalized citizens,” who were born abroad. About 22,000 citizens were “denaturalized” that century, legal scholar Patrick Weil noted in his book “The Sovereign Citizen.” Another 120,000 U.S.-born citizens who were living abroad were stripped of their citizenship over the same period.

Nobody should be allowed to burn the American flag – if they do, there must be consequences – perhaps loss of citizenship or year in jail! — Donald J. Trump (@realDonaldTrump) November 29, 2016





American-born U.S. citizens could lose their citizenship if they served in foreign armies, voted in foreign elections or were found guilty of treason or desertion, thanks to a 1940 law passed as fears mounted over World War II. Decades before that, Congress decided naturalized citizens born abroad could be stripped of citizenship for speaking out against the government during wartime or belonging to “un-American” political groups or even ethnicities. (The government began a campaign to denaturalize Asian Americans in 1906.) Emma Goldman, an anarchist, was the first person in the US to lose her citizenship due to her political affiliations in 1908.

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(The United States wasn’t the only country denationalizing people in the 20th century. The Soviet Union denaturalized 1.5 million citizens between the world wars.)

The Supreme Court slowly curtailed the government’s ability to strip citizenship from foreign-born Americans, taking up the subject a number of times between World War II and the 1970s. The court struck down the revocation of citizenship of a German-born American accused of sympathizing with the Nazis and later upheld the citizenship of the leader of a Communist group in California.

When it came to taking away citizenship from American-born citizens, the court took a firmer stand. According to Weil’s book, Chief Justice Earl Warren wrote that suggesting the government could simply reject U.S.-born citizens would have been “shocking” to the Founding Fathers. Warren eventually convinced enough fellow justices to back him based on the 14th Amendment’s definition of citizenship and the Eighth Amendment’s prohibition of cruel and unusual punishment.

Slideshow: Flag-burning in America: A look back >>>

In Afroyim v. Rusk (1967) the court held that the 14th amendment protected both native-born and naturalized citizens from being denaturalized. A 1971 decision narrowed that finding a bit, upholding Congress’ right to strip citizenship from people born to a U.S. parent abroad under certain circumstances.

It’s unclear whether the current court would agree with the 50-year-old precedents that define our current understanding of U.S. citizenship as an irrevocable right. Both of the foundational cases were won with slim 5-4 majorities.

“It’s fair to say that the Supreme Court hasn’t visited this in a long time,” said Jeffrey Rosen, a law professor and the president and CEO of the National Constitution Center. (Rosen added that in the specific case of flag burning, he suspected that the current court under Chief Justice John Roberts would uphold it as a First Amendment right.)

The court may be forced to reconsider the issue as Trump is not the only Republican to recently suggest revoking citizenship as punishment. In 2014, Sen. Ted Cruz introduced a bill called the “Expatriate Terrorist Act” that would allow the government to strip people of their citizenship if they provide “assistance” to a terrorist group.

Weil, the legal scholar who wrote a book on denationalization, said the most telling example of how protected U.S. citizenship has been in recent decades is that of Dzhokhar Tsarnaev, the Boston Marathon bomber who became a naturalized citizen just a year before his attack. “The federal government requested the death penalty but not denaturalization,” Weil said. “They could not because of this precedent.”