The Pentagon’s latest annual report on Chinese military power serves a reminder that the world has embarked on a second nuclear age, following the first one that began at Hiroshima and Nagasaki 75 years ago. The good news is that arms-control accords slashed the number of weapons built for the Cold War. The bad news is that the nuclear club now includes far more countries than before. New members come in many shapes and sizes, with varying economic and military potential. Some border one or more potential antagonists. Some newcomers are building up their inventories while old-timers from the first nuclear age cut back or hold them steady.