It’s been a rough few weeks for the North Atlantic Treaty Organization. A bedrock of the liberal international order, NATO has over the years faced challenges and crises from both within and without. But until recently, few would have envisioned NATO becoming a punching bag for the president of the United States—the nation that was its architect and for seven decades its principal leader—while that president moves publicly closer to Russia.

NATO’s first secretary general, Hastings “Pug” Ismay, famously declared that the organization’s mission was “to keep the Russians out, the Americans in, and the Germans down.” It’s a wonderful line that encapsulated 75 years of brutal history and tragically learned lessons. And it’s worth reexamining Ismay’s framework through this prism to understand just how, exactly, NATO was intended to ensure just that.