This work outlines a strategy for how the contemporary left should build progressive alliances. It takes a ‘drawing lessons from history’ approach, focusing especially on the exchanges of ideas that took place among European progressive movements and authors between WW1 and WW2. These exchanges bridged ideological and partisan divides between socialists and liberals of various stripes, and included prominent British figures—e.g., John Maynard Keynes, Clement Attlee, John A. Hobson, G.D.H. Cole, and Ramsay MacDonald— as well as their contemporaries in Austria, the Benelux, and Weimar Germany. This work seeks to connect these interwar debates to the questions raised by the current crisis in social democracy, and the somewhat contrasting backdrop this provides to the Labour party’s recent electoral and polling positions. Rather than letting the relationship between ‘radicals’ and ‘moderates’ on the left in current political discourse be defined by the ‘reformist’ or ‘revolutionary’ nature of their aims, the work makes the case for today’s social democrats to adopt a cross-party and cross-class ‘unity strategy’.