Today more than ever before, early post-war European social democracy presents historians with a conundrum. Emerging from the Second World War euphoric at the prospects of a rapid breakthrough to socialism, European social democrats soon found their high hopes disappointed. Towards the end of the decade, they had been driven onto the defensive across the continent. In Western Europe, social democratic parties had either become junior partners in coalitions dominated by the Christian democrats or were on their way out of government altogether. In Eastern Europe things were worse still, as social democratic parties were browbeaten into mergers with the communists – not to return to the political scene before the collapse of the Iron Curtain. Where had it gone wrong?