Leftist violence and plausible deniablity.

Richard Fernandez makes the point that

In fine anarchists are Red Guards of the European Left, a collection of dupes formed inside the vast and creaky infrastructure of Marxism to advance the interests of one faction against another faction. It is impossible to understand the politics of the Left without grasping that it is all about deniable intimidation. The real problem European anarchism solves is how to send bombs without seemingly sending them, or how to trash the Tory party headquarters in London without really doing it.

That's true whether describing the rampaging "anarchists" in London, or Madison, Wisconsin public employee unionists sending death threats to people who disagree.