A new wave of optimistic, radical young people were building mass movements outside the political bubble, rejecting the neo-liberal consensus and the enrichment of the 1%, and laying the foundations of a new left. Eight years later, Bernie Sanders was favorite to clinch the Democratic Party nomination, and Jeremy Corbyn and Momentum had taken over the Labour Party in Britain, promising ‘a new kind of politics’. But as the new left poured into Labour, it was overwhelmed by older, institutional forces on both left and right. Four years after Corbyn became leader, after bitter-infighting and a Brexit-fuelled strategic crisis, it all fell apart. This is the inside story of how the left came back to life in the 2010s, from a man who found himself at the centre of events, featuring unparalleled access and a range of interviews with key left-wing figures. Influential journalist and activist Michael Chessum explains how this movement was built, why it failed, and what it needs to do now.