Ask someone to explain what “revolution” is, and there’s a decent chance they will reply that, though they can’t offer a precise answer, they know it when they see it. This nebulousness, according to Donald Sassoon, is innate to the concept. No “strong definition” is possible; at best, like “most complex phenomena,” it can only be classified “in generic terms.” This seeming insuperability hasn’t prevented a legion of scholars from writing about revolution, either as a category or its innumerable manifestations. Revolutions: A New History, Sassoon’s survey of the subject, is the latest addition to the genre. Acerbic, informative, opinionated, deeply researched, and surprisingly fun, it’s a welcome one. And if by the end he still can’t tell you exactly what a revolution is, like his readers, he knows one when he sees one.