World War II, the atomic bomb, the Cold War, made it hard for Americans to continue their optimism.

Ambrose is pointing to a mid‑20th‑century rupture in the American habit of expecting progress. The scale of World War II’s destruction, the moral and existential shock of nuclear weapons, and the long, anxious standoff of the Cold War introduced a persistent sense of vulnerability and limits—technological power could now end civilization as easily as improve it. In this reading, “optimism” is not mere cheerfulness but a national narrative of steady advancement; these events complicated that narrative by making catastrophe feel plausible and permanent. The line fits Ambrose’s broader interest in how large historical forces reshape American self‑understanding and civic confidence.