Terrorism has a long history in the United States going back to the nation's founding when westerner farmers used terror to intimidate federal tax officials attempting to collect an excise tax on whiskey during President Washington's administration. Terror has not always been perpetrated by dissendents, some of it is state sponsored. Consider the events at Kent State University on May 4, 1970 when Ohio National Guardsman fired 67 rounds in 13 seconds on unarmed students. Four students were killed, nine wounded, including one student paralyzed for life. There was an order to fire, but the civil rights case against individual guardsmen was dismissed. This is an amateur video:
If you believe the Vietnam war protests are too far in the past to be relevant to a discussion of increasing authoritarianism in the United States then look at this brief compilation of footage by Whispered Media from the November, 1999 protests around the WTO meeting in Seattle: