(NBC NEWS) — After decades of relative obscurity, the fringe "antifa" movement is becoming a household name after followers clashed with white supremacists at the Charlottesville "Unite the Right" rally where extremist Alex Fields is accused of murdering 32-year-old activist Heather Heyer in a car attack.

But the movement is still loosely defined and organized, making it difficult to get a grip on its size and aims.

Professor Mark Bray, a historian and lecturer at Dartmouth, has tried to fill the gap in his new book, "Antifa: The Anti-Fascist Handbook," that chronicles its rise. While Bray doesn't participate in the group's protests, he nonetheless considers himself an ally.

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