It is, of course, unlikely that chattel slavery will return to the US, or that genocide against Jews and wholesale murder of LGBT people will happen in the US, but history has made it clear that there are never any guarantees about questions like these. Given that, a strong argument can be made not just that occasional violence can be excused in response to these symbols, but that violence is the only rational and morally acceptable response. That may sound extreme, but it was not that long ago that young American men were required to violently oppose Nazis, while those who refused were called traitors.