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.

President Obama acknowledged that danger at the end of the Nuclear Security Summit meeting in Washington early this month. He warned of the potential for “ramping up new and more deadly and more effective systems that end up leading to a whole new escalation of the arms race.”