In America, police officers generally do not use flamethrowers, and are not trained to use them. Flamethrowers would do a lot of damage to the buildings, and even people of DC.
I have noticed that Alt Right terrorists usually have far less military experience than they claim. They have no understanding of air support. A gun airship could usually eliminate them quickly, if it were to come to that. They are doing some amazing things with drones, so with a one or two thousand dollars any technically adept Antifa person could also do the same.
Please Walt, enthrall me with all your military derring do.