BERLIN—About 15 leftist radicals gathered over stale coffee on a recent Friday night in the German capital to plan their next communist plot.

"We must take care not to sound elitist, as if we're outside the mainstream, because the mainstream is the proletariat," said a young man dressed in black clothes, a tattered green copy of Friedrich Engels's 1884 book, "The Origin of the Family, Private Property and the State," at his side.

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