The socialist contribution to the creation of the European Economic Community has long been overlooked and misunderstood. Existing scholarship emphasises short-term considerations in explaining why the French Socialist and German Social Democratic Parties supported a European Common Market in 1956–7. This article offers a new perspective by placing these parties’ decisions within a longer context of socialist views on free trade, tariffs and regional economic organisation. Based on fresh archival materials, this article explores how socialist proposals for securing an economic peace after the First World War continued to influence socialist policies on European economic integration in the 1950s.