LONDON — A veteran anti-war campaigner known for his unapologetically socialist views has won a landslide victory to lead Britain's opposition Labour Party — an outcome that delighted supporters and dismayed others who never imagined he could be elected. The overwhelming support for far-left lawmaker Jeremy Corbyn, 66, was one of the biggest shake-ups in British politics in decades. His win Saturday marks a sharp left turn for his party — and will significantly challenge the tone of mainstream British politics, which is now dominated by Prime Minister David Cameron's Conservatives. "It's certainly the most high-profile position an overt socialist...