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.