European countries, after the Second World War, came to a painful but profound conclusion: if they wanted to stop centuries of bloodletting among themselves, they had to make war materially irrational. The answer they found was not idealism alone, but interdependence. Trade. Shared markets. Shared interests. Over time, that logic helped give birth to what became the European Union. As the EU notes in its official history of postwar integration, the European Coal and Steel Community was created so that no single country could build the weapons of war against the others as in the past. Established by the Treaty...