It is extraordinary how few countries can build nuclear weapons. Eighty-one years have passed, almost to the day, since the bombing of Hiroshima. Yet, of the 195 states recognized by the United Nations, 186 are technologically unable to make warheads. Beyond the original five nuclear states — that is, the five countries that sit on the U.N. Security Council — only four have unofficially acquired the capability: India, Pakistan, Israel, and North Korea. That’s it. More nations have discontinued nuclear programs than have gotten the Bomb.

“I am haunted by the feeling that, by 1970, there may be 10 nuclear powers instead of four,” said President John F. Kennedy in 1963, in the aftermath of the Cuban Missile Crisis, “and by 1975, 15 or 20.” In fact, largely through U.S. efforts to deter foes and reassure allies, the number is still in single figures.