The Shadowy Extremist Group Behind the Anti-Trump RiotsAntifa, which stands for “anti-fascist action,” is a network of loosely affiliated
far-left anarchist and communist groups that orchestrate violent protests and attacks on populists, conservatives, and anyone else its members deem to be “fascists” or “Nazis.”
Antifa was formed originally in Germany in the 1980s,
its members taking the name of the communist paramilitary groups that engaged the Nazis in street-fighting in the 1930s. It now has
active cells across the world, including in Germany, the U.S., Canada, the Netherlands, Norway, Sweden, Czechia, and France.
“Anyone who tries to hold any sort of right-wing event
literally gets beat up by militant communists in the street,” said independent journalist Lauren Southern, who was present at one of the pro-Trump Berkeley rallies, in a video taken immediately following the violence.

But some Antifa members have clearly had the same thought — that "flags and bats" simply aren’t enough. Indeed the
Antifa United webstore temporarily sold an Antifa-branded concealed credit-card knife.
The Department of State, through U.S. Code Title 22, Chapter 38, defines terrorism as "premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents."
Antifa increasingly could plausibly fit that definition.
The U.S. Department of Defense defines terrorism as "
the unlawful use of violence or threat of violence to instill fear and coerce governments or societies" and notes that terrorism "is often motivated by religious, political, or other ideological beliefs and committed in the pursuit of goals that are usually political."
Yes, Antifa fits that definition, too.
http://www.lifezette.com/polizette/the-shadowy-extremist-group-behind-the-anti-trump-riots/