Hello and welcome back. This is WAR 1870–1949: How Empires Rise and Fall. Episode Four of our buildup arc is The Victors in Fear: Britain and France Hold the Line.

If you want to understand the interwar period properly, you have to strip away the victory photographs. The leaders look composed. The flags look proud. The map looks controlled. But underneath, Britain and France are not walking into peace—they’re walking into fragility. They have won, and they are terrified. Not theatrically. Structurally. Because they can feel, in their bones, that the old European order has been damaged beyond repair, and they don’t know what will replace it.

France has been invaded and scarred. The battlefield has been on its soil. Whole regions have been torn up. A generation of young men is missing. That absence is not poetic; it’s demographic. It’s  ...