This weekend, we saw the tragic eruption once again of political violence in America. Unfortunately, this is just the extreme end of a growing wave of threats, intimidation and occasionally worse that is hitting public officials at all levels. While America has gone through waves of more and less violent periods in its recent past, this wave is distinctive, and it is very much coming as part and parcel of a new authoritarian moment. Such times call for clear heads and careful analysis, so let's make a start.