Of all Trump's departures from tradition, none holds such potentially grave consequences as his decision to build up U.S. nuclear weapons. After a decades-long trend toward disarmament, Trump's estimated $1.2 trillion-dollar upgrade would not only make current nuclear bombs more lethal but would add new long-range missiles to the U.S. arsenal.

When Trump announced his plan for nuclear weapons "modernization" in his State of the Union address, he acted as if the rationale were so obvious that any six-year-old would get it. Perhaps he's right, in that six-year-olds might think that the side with the biggest, deadliest bombs wins. But while that may have been true of warfare in the past, most grown-ups who've given the issue thought — physicists, arms-control experts, and several former war planners — know that the nuclear age is different.

The rules of the game of war suddenly changed in the mid-20th century, when the U.S. and the Soviet Union both acquired the ability to instantly destroy the world hundreds of times over. At that point, survival and security became less a matter of technology and more one of psychology. The ostensible reason the United States continues to keep nuclear weapons is to deter other nuclear states from using theirs.