Jeremy Corbyn is perhaps the most radical politician to head the British Labour Party in half a century. He lacks personal charisma, a significant record of legislative achievement, and any realistic prospect of leading his party to victory over the Tories in a parliamentary election. His views on world politics—a mix of praise for the UN, support for disarmament, opposition to global capitalism, and a tepid attitude toward the EU—appeal to an archaic version of British leftism. This book by a Marxist British journalist seeks to explain and justify the enthusiastic support that Corbyn has gained from many Labour Party members. Corbyn, Seymour argues, is a spokesman for those left behind by the twenty-first-century economy, similar in that respect to Marine Le Pen, Bernie Sanders, and Donald Trump. Corbyn may sometimes seem diffident and even hapless, but the United Kingdom needs a “movement politician” who can make use of social media and assemble large rallies of the faithful. Seymour’s book is by turns inspiring and implausible. Yet it is required reading for those trying to puzzle through the rebirth of more extreme parties in Western democracies.