On Sunday, President Trump tweeted that the United States will be declaring Antifa a terrorist organization in response to the ongoing civil unrest across the country.

His tweet has raised numerous legal and constitutional questions. Although the President can use an executive order to label individuals or groups as terrorists, the very nature of the Antifa movement makes that rather tricky. Moreover, this is uncharted legal territory as there is no existing legal authority to designate a domestic organization as a terrorist one, according to Mary McCord, a former Acting Assistant Attorney General for National Security at the Department of Justice.

But what exactly is Antifa?

The Antifa movement can trace its roots back in the 1930s in mid-war Germany. Then, the German Communist Party has established the Antifaschistische Aktion as a counterbalance to the Nazi Party and its Sturmabteilung, better known as the Brownshirts.

The movement saw a resurgence in the 1980s but with an important ideological difference: The initial Antifa movement, being a spawn of the German Communist Party, prescribed to Marxism-Leninism. The new version of the movement, however, leaned more towards Libertarian Marxism and Anarcho-Communism, while still maintaining some elements of Marxism-Leninism, such as the abolition of private property and wage labor.

The Antifa ideology focuses on concepts rather than people. For example, it calls for the abolition of organized states, economic systems, like capitalism, and private property, among other things. The willingness to utilize violence to achieve this distorted end state is what makes Antifa dangerous. These are not your Woodstock hippies who will sing away their “enemies;” no, these are believers who will not shy away from violence in order to achieve their goal. To justify the use of violence, the (modern) Antifa ideology argues that opposing fascism and other oppressive ideologies, which are inherently violent, is a form of self-defense.

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In theory, that argument is not entirely illogical. In reality, however, Antifa groups have a tendency to label anyone who opposes them or disagrees with their positions as a fascist, thereby justifying violence in pretty much every case. Having the authority to adjust your rules of engagement as you see fit can be really dangerous. In what is happening today in America, Antifa groups see the reaction of the law enforcement departments and the National Guard against the ongoing civil unrest as oppressive, thus justifying for the use of violence against them.

The American version of Antifa began in the 1980s as a response to the activity of some neo-Nazi groups. It opposes white supremacist, fascist, and racist groups. More recently, it has also come to oppose the alternative-right. The movement was rekindled ever since Donald Trump became the president of the U.S.

In Europe, the Antifa movement is far more established and experienced. During the euro economic crisis that rocked the European Union, it was a common sight to be seeing Antifa members fighting it out with the police during protests. Greece, in particular, became a battlefield where Antifa members from other European countries, most notably from Italy, Germany, and Austria, were traveling to participate in the riots. Moreover, there has been a historical connection of the Antifa movement with a lot of soccer clubs — such as the German St. Pauli, the French Marseille, or the Belgian Standard Liege — and their hooligans.

Unlike most political or terrorist organizations, Antifa does not have a centralized structure: It is rather comprised of small autonomous groups that share a common ideology. That is one of the reasons why labeling Antifa a terrorist organization would be tricky.

It should be noted that both the Democrats and Republicans are opposed to the Antifa movement. Indeed, Speaker of the House Nancy Pelosi had called for the prosecution of some Antifa members after they had succeeded in canceling a series of speeches in Berkeley, California, in 2017.