WASHINGTON, D.C. President Donald Trump tweeted Sunday afternoon that the United States will designate antifa as a terrorist organization, although some say the U.S. government doesn't have the legal authority to do so.

The State Department can designate foreign organizations as terrorist groups, but the United States has no domestic terrorism statute, according to the American Civil Liberties Union.

"Let's be clear: There is no legal authority for designating a domestic group," the ACLU tweeted Sunday. "Any such designation would raise significant due process and First Amendment concerns."

On Tuesday, Twitter announced that a white nationalist group had been posing as antifa, presumably to cause dissension.

For those who might be a little unsure as to what and who antifa is, here are five things to know about the movement of militant activists.

1. What does Antifa stand for and what are their general beliefs?

Antifa, short for anti-facists, is an umbrella description for a broad group of people whose political beliefs often fall to the far left but do not conform with the Democratic Party.

Antifa members stand against what they view as authoritarian, homophobic and racist systems, according to The New York Times.

2. How long has antifa existed?

The original antifa groups date back to fights against European fascists in the 1940s. The modern antifa movement in America began in the 1980s with a group called Anti-Racist Action, according to the "Antifa: The Anti-Fascist Handbook."

3. Who is in antifa?

Part of the issue with Trump's claim that he will designate antifa as a terrorist organization is that it's difficult to label antifa as an organization at all.

The movement has no official leaders or headquarters. Over the past decade, antifa has worked with other local activist networks that are rallying around shared beliefs, such as Black Lives Matter, but it's impossible to know how many members there are, according to The New York Times.

4. What does antifa protest, and what are its tactics?

Antifa members take part in protests and rallies aimed at disrupting authoritative speakers and actions. Many antifa organizers participate in peaceful forms of community organizing, but what sets the group is apart is its willingness to use violence.

Antifa members say they use violence as a means of self-defense and that property destruction does not equate to violence, according to CNN.

"There is a place for violence," Scott Crow, a former Antifa organizer, told CNN. "Is that the world that we want to live in? No. Is it the world we want to inhabit? No. Is it the world we want to create? No. But will we push back? Yes."

5. Why do antifa members dress in all black?

Antifa members will often dress in all black, sometimes also covering their faces with masks, so they can't be identified by opposing groups or the police.

The all-black uniform is also an intimidation tactic, which allows members to move through a protest as one uniformed group.

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What Is Antifa: 5 Things To Know About The Movement