Is the world on the brink of another nuclear arms race?

The Japanese city of Nagasaki held a somber ceremony on Sunday to mark the 75th anniversary of the U.S. atomic bombing that killed 70,000 people in the waning days of World War II. The attack came just three days after the American military bombed Hiroshima, killing 140,000.

These remain the only instances in which nuclear bombs have been used in war, but the specter of an apocalyptic conflict has hovered over international conflicts ever since. After a series of close calls in the 1950s and 1960s that led to the...