The United States, Russia and China are now aggressively pursuing a new generation of smaller, less destructive nuclear weapons. The buildups threaten to revive a Cold War-era arms race and unsettle the balance of destructive force among nations that has kept the nuclear peace for more than a half-century.

It is, in large measure, an old dynamic playing out in new form as an economically declining Russia, a rising China and an uncertain United States resume their one-upmanship.

American officials largely blame the Russian president, Vladimir V. Putin, saying his intransigence has stymied efforts to build on a 2010 arms control treaty and further shrink the arsenals of the two largest nuclear powers. Some blame the Chinese, who are looking for a technological edge to keep the United States at bay. And some blame the United States itself for speeding ahead with a nuclear “modernization” that, in the name of improving safety and reliability, risks throwing fuel on the fire.