When Donald Trump threatened millions of unauthorized immigrants with deportation on Wednesday night, he turned to a phrase he's used time and again during his campaign: "America First."

"We need a system that serves our needs, not the needs of others," he said Wednesday night. "Remember, under a Trump administration it's called America first. Remember that."

For students of history, that’s more than just an anodyne slogan. It also happens to be a phrase with a long, sordid backstory in US politics — and one that Donald Trump has somewhat strangely adopted during his campaign.

In 1940, "America First" referred to a group that resisted America’s entry into World War II before Pearl Harbor. The cause eventually came to be associated with not just antiwar objectors but also virulent anti-Semites, and the term itself became somewhat taboo. In the decades since, politicians have mostly shied away from the phrase, with a few exceptions on the fringe like Pat Buchanan.

Then came Donald Trump. He’s happily seized on an expression that once stood for isolationism and xenophobia and turned it into one of his many vague, ebullient catchphrases. Much like "Make America Great Again!" Trump uses "America First!" as an exclamation point to sum up everything from energy policy to his support for veterans.

He's either unaware of its historical implications or chooses to ignore them, telling the New York Times in July that he knew about the history but uses it as a "brand-new, modern term."

But the disquieting history of how "America First" eventually became a byword for anti-Semitism is very relevant to the Trump campaign. The America First movement attracted many of the same kinds of people drawn to Trump, including racists and bigots empowered by seeing their views reflected in a national debate.

And by not disowning its worst supporters — something Trump has also been criticized for failing to do — America First ensured that their acts became its historical legacy.

"America First" started with skepticism about a wasteful war

The original "America First" movement started in the bitter wake of World War I, the deadliest war for the United States since the Civil War — killing more than 116,000 Americans.

After the war ended, little seemed to change in European politics. And many Americans were furious that their boys had died for nothing. A popular narrative took hold that the British, the French, and the defense industry together had duped the United States into wasting its resources and the lives of its young people. Companies that made and sold weapons became known as "merchants of death."

This was far from a fringe view. A 1936 report by a bipartisan Senate committee declared that enriching arms manufacturers was the major cause of the war. By 1937, 70 percent of Americans thought fighting in World War I was a mistake, according to Gallup.

So in the late 1930s, when another war in Europe loomed, the United States was deeply divided over the prospect. Many war opponents questioned whether Adolf Hitler was really a threat to the United States and whether the British were really allies worth helping.

This isolationist movement was particularly strong on college campuses. College students who grew up in the years after World War I found jingoistic patriotism to be outmoded. One University of Minnesota student called it "a cheap medallion with which to decorate and justify a corpse," according to author Lynne Olson in her history of the period, Those Angry Days: Roosevelt, Lindbergh, and America’s Fight over World War II.

The debate was particularly emotional on Ivy League campuses; in 1940, students tried to shout down a commencement speaker who argued in favor of intervention in Europe. The America First organization was born at Yale. Two future US presidents — John F. Kennedy and Gerald Ford, then both still in college or graduate school — were early supporters.

College students who thought patriotism was a sham are perhaps not what Trump hopes to invoke when he tweets, "America First!" with a picture of one of his rallies. But the emotions that inspired the original America First committee aren’t exactly absent in 2016, either. Trump has made his supposed opposition to the Iraq War a key argument in favor of his candidacy. Many of his arguments are calculated to appeal to Americans who think they are getting a bad deal from the rest of the world.

Yet what happened next to the America First movement shows how those sentiments can lead to very dark places.

America First refused to disown its ugliest supporters — and became defined by them

Eventually, once the America First movement became a national organization, headquartered in Chicago, college students started to fall away in favor of conservative business leaders who wanted to stick it the liberal president they hated and Midwesterners who felt that East Coast elitists were condescending to them.

A wide variety of bigots also joined the cause. The America First movement didn’t create violent hatred of "the Other" in America, but it did provide a release valve for those sentiments where they existed — creating a cultural clash similar to fights between Trump’s supporters and his opponents.

The period after World War I provided plenty of fodder for anger for social and racial conservatives, not least once the Great Depression struck. The late 1930s were rife with blatant displays of overt racism and xenophobia against immigrants and Jews. Since immigration of Eastern European Jews had dramatically increased until the US started restricting immigration through quotas in 1924, the two groups were often one and the same.

One group, the Vindicator Association, called for young people to form vigilante "border patrols" to stop "alien criminals." A member of Congress said that America should "close, lock and bar the gates of our country … and then throw the keys away." An overwhelming majority of Americans wanted to reject Jewish refugees from Europe. Anti-Semites accused FDR of being secretly Jewish.

All of this hate found an outlet in the America First campaign. Industrialist Henry Ford, one of the most prominent supporters of anti-Semitic conspiracy theories, was a supporter. The anti-Semitic radio host Father Coughlin, whose followers sometimes attacked and beat up Jewish people, urged his audience to join America First, and they did. America First opponents and supporters would show up to competing rallies that turned into fights, with isolationists yelling, "Jews!" and internationalists yelling, "Nazis!"

The problem got much worse when Charles Lindbergh, the aviation hero and prominent isolationist, gave a speech for the group in which he accused Jews of pushing America into war and said their influence was particularly pernicious because Jews controlled government and the media.

An endorsement from an aviator who had been hailed as a hero in the 1920s was incredibly empowering for anti-Semites in America, who wrote thousands of supportive letters to America First. Lindbergh fell from grace almost immediately; the media, even newspapers that supported isolationism, turned on him. As Olson documented:

Before Lindbergh, [Liberty magazine] wrote, "leaders of anti-Semitism were shoddy little crooks and fanatics sending scurrilous circulars through the mails.… But now all that is changed.… He, the famous one, has stood up in public and given brazen tongue to what obscure malcontents have only whispered."

Lindbergh gave his speech on September 11, 1941. By then, as more news had emerged from Europe, more Americans favored intervening in the war than had a year earlier anyway. But Lindbergh’s embrace of anti-Semitism, combined with his already-established penchant for saying nice things about Nazi Germany, tarnished his legacy permanently. Even anonymously fighting in combat during World War II didn’t rehabilitate him in the eyes of Americans.

What Trump could have learned — but didn’t — from America First

America First has become historical footnote partly because it was a lost cause — interventionists decisively won the debate about World War II after Pearl Harbor — but also because anti-Semitism in the US became much less socially acceptable after the scope of the Holocaust was fully known.

The phrase "America First" was intermittently resurrected in the decades since, but the odor of anti-Semitism never went away. When Buchanan employed the phrase in 1991, he also used an updated version of Lindbergh’s old anti-Semitic argument, saying supporters of Israel — Jews — were trying to start wars they wouldn’t fight in.

Donald Trump seems ignorant of all these historical undercurrents. He sidestepped that controversy by watering down the meaning of America First to a synonym for "Make America Great Again!" Just after he called for a foreign policy to put America first in his April speech, he went on to say that "in the 1940s, we saved the world" — a key tell that he wasn’t really interested in any World War II-era connotations.

When the New York Times's David Sanger pressed him on his use of the phrase in July, Trump shrugged off the historical parallels:

SANGER: We talked about that a little bit at the last conversation. Does America First take on a different meaning for you now? Think about its historical roots. TRUMP: To me, America First is a brand-new modern term. I never related it to the past. SANGER: So it's not what Lindbergh had in mind? TRUMP: It's just, no. In fact when I said America First, people said, "Oh, wait a minute, isn't that a historical term?" And when they told me, I said: "Look, it's America First. This is not ——" SANGER: You were familiar with the history of the phrase. TRUMP: I was familiar, but it wasn't used for that reason. It was used as a brand-new, very modern term.

But there are parallels beyond the specifics of foreign policy here.

Trump has his own problems with supporters who are usually ostracized from mainstream American politics. He was slow to denounce KKK leader David Duke, who has continued to praise Trump's campaign. He turned a meme that imposed a Star of David and Hillary Clinton’s face on top of a pile of money, widely perceived as anti-Semitic, into a weeklong controversy when he refused to disown it or apologize for it. He hired Steve Bannon, whose website Breitbart harps on crime by black Americans and Hispanic immigrants, to run his campaign.

Trump could learn something from America First — which reacted to Lindbergh’s universally condemned speech with a weak statement saying that the speech wasn’t anti-Semitic — about what happens to movements that don’t police and disown their unsavory supporters.

What Donald Trump means by "America First"

Trump first showcased the term "America First" in a foreign policy speech back in April, in which he declared that trade agreements, permanent alliances, and immigrants were burdens weakening America rather than the bonds that reinforce international peace.

"‘America First’ will be the major and overriding theme of my administration," he said in his speech. To him, that meant disconnecting from other countries: more barriers to trade, tougher negotiations with longstanding allies in NATO, and a more restrictive immigration policy.

This view of the world — the first time a serious contender for the Republican nomination had called for retreating from the world since 1952 — came in for harsh criticism from various foreign policy experts. But Trump didn’t back down. Quite the opposite: He became so taken with the phrase "America First" that he began applying it to other policies, like his energy plan:

Then he just turned it into a hashtag, one that takes fewer characters than #MakeAmericaGreatAgain:

Meanwhile, "America First" night at the Republican National Convention showcased a diverse range of speakers without a single unifying message. It’s no longer even clear what "America First" means to Trump. But it’s pretty clear that doesn’t care about the phrase’s historical weight — even when it seems very relevant to his own campaign.