In the weeks since President Donald Trump issued an executive order titled “Designating Antifa as a Domestic Terrorist Organization,” he and his administration have repeatedly and forcefully sought to describe left-wing protesters, both peaceful and violent alike, as domestic terrorists. But the question of who actually constitutes antifa and the legal and political consequences of Trump’s executive order remain murky. For starters, antifa, which is short for anti-fascist, is an ideology whose adherents are not part of any unified organization. And more importantly, experts said Trump has no legal authority to designate a group as a domestic terrorist organization, since...