From 1947 until 1990, the United States faced a mortal threat, an enemy that was strategic, political, military and ideological. Washington had to keep together an alliance that faced up to the foe and persuaded countries in the middle not to give in. This meant that concerns about resolve and credibility were paramount. In this context, presidents had to continually reassure allies. This is why Dean Acheson is said to have remarked in exasperation about Europe’s persistent doubts about America’s resolve, “NATO is an alliance, not a psychiatrist’s couch!”
But the world today looks very different — far more peaceful and stable than at any point in decades and, by some measures, centuries.