Leading up to 1914 socialist movements all over Europe, notably in France and Germany, had become so strong that they were in the very halls of power. But when faced with the onset of the Great War, the established socialists blinked, unwilling to risk their heard-earned position and possibly be arrested and have their parties driven underground again. This is precisely what they should have done, argued Lenin in 1915. We discuss this possible missed opportunity – why the socialists failed to stop the war before it started.