There’s a strange monument to European unity on Barcelona’s Montjuic hill: big stone slabs, with a quote from the Treaty of Rome in Catalan, and the dates of birth and death for a series of European notables, ranging from founding fathers of the European Union like Jean Monnet, to national politicians like Winston Churchill, Willy Brandt, and Francois Mitterrand.

But there’s a surprising addition to their ranks: Rosa Luxemburg. The inclusion of this Polish-German revolutionary would no doubt have bemused her contemporary Churchill, who denounced Luxemburg for belonging to a “sinister conspiracy” of “international Jews,” drawn from “the underworld of the great cities of Europe and America,” who were seeking to overthrow Western civilization.

The gulf between Luxemburg and Churchill was scarcely greater than the gulf between her political vision and that of the present-day European Union. However, there’s one group in the European parliament that claims direct descent from Luxemburg’s ideas: the various strands of Europe’s radical left. Is their position at the heart of the European project as incongruous as Luxemburg’s presence on Montjuic? Or can the radical left somehow bend that project to its purposes?