It is largely 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.

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The United States, Russia and China are 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 50 years.

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

U.S. officials largely blame the Russian president, Vladimir 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 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 in Washington, D.C., 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.”

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For a president who came to office more than seven years ago talking about eventually ridding the world of nuclear weapons, it was an admission that a U.S. policy intended to reduce the centrality of atomic arms might contribute to a second nuclear age.

One of the few veterans of the Cold War in his administration, James Clapper, director of national intelligence, told the Senate Armed Services Committee during his annual global threat assessment, “We could be into another Cold War-like spiral.” Yet it is different from Clapper’s earlier years, when he was an Air Force intelligence officer weighing the risks of nuclear strikes that could level cities with weapons measured by the megaton.

Adversaries look at what the United States expects to spend on the nuclear-revitalization program — estimated at up to $1 trillion over three decades — and use it to lobby for their own sophisticated weaponry.

Russia is fielding big missiles topped by miniaturized warheads, and experts fear it may violate the global test ban as it develops new weapons. According to Russian news reports, the Russian navy is developing an undersea drone meant to loft a cloud of radioactive contamination from an underwater explosion that would make target cities uninhabitable.

The Chinese military, under the tighter control of President Xi Jinping, is flight-testing a novel warhead called a “hypersonic glide vehicle.” It flies into space on a traditional long-range missile but then maneuvers through the atmosphere, twisting and careening at more than a mile a second. That can render missile defenses all but useless.

The Obama administration is not in a position to complain. It is flight-testing its own hypersonic weapon, but an experiment in 2014 ended in a spectacular fireball. Flight tests are set to resume next year. As part of the modernization process, it is also planning five classes of improved nuclear arms and associated delivery vehicles that, as a family, are shifting the U.S. arsenal in the direction of small, stealthy and precise.

“We are witnessing the opening salvos of an arms race,” James Acton, a senior analyst at the Carnegie Endowment for International Peace, last year told a congressional commission that assesses China’s power.

One fear about the new weapons is that they could undercut the grim logic of “mutual assured destruction,” the Cold War doctrine that any attack would result in massive retaliation and ultimately the annihilation of all combatants. While much debated and often mocked — in classics such as the movie “Dr. Strangelove” — MAD, as it was known, worked. Now, the concern is that the precision and less-destructive nature of these new weapons raises the temptation to use them.

A key question that Obama addressed is whether America’s planned upgrades are helping drive this competition. Or are Russia and China simply using the U.S. push as an excuse to perfect weapons they would build anyway?

Russia and China, analysts say, are testing space weapons that could knock out U.S. military satellites at the beginning of a nuclear war. In response, the U.S. is launching space-observation satellites meant to deter and help defeat such attacks.

Obama, speaking at the summit’s closing news conference, acknowledged the tension stirred by the refurbishment of the nation’s aging nuclear arsenal. He noted, for example, that communication links between the weapons and their guardians needed better protections against cyberattack. But when asked if warhead miniaturization and similar improvements could undermine his record of progress on arms control, he replied: “It’s a legitimate question. And I am concerned.”

William Perry, defense secretary under President Clinton and one of the most influential nuclear experts in the Democratic Party, said he worried that Russia would soon withdraw from the Comprehensive Test Ban Treaty of 1996 and begin perfecting new warheads in underground detonations. (The United States has abided by the treaty, but the Senate has never ratified it.)

For two decades, the main nuclear powers have observed a shaky global ban on testing, a central pillar of nuclear arms control.

“I’m confident they’re working on a new bomb,” Perry said in an interview, referring to Russian nuclear-arms designers. “And I’m confident they’re asking for testing.”

“It’s up to Putin,” he added.

Advocates of the U.S. nuclear-modernization program call it a reasonable response to Putin’s aggression, especially his 2014 invasion of Crimea.

Military experts argue that miniaturized weapons will help deter an expanding range of potential attackers. “The United States needs discriminate nuclear options at all rungs of the nuclear escalation ladder,” said a report last year from the Center for Strategic and International Studies, a research group in Washington.

In February, the Obama administration backed development of an advanced cruise missile. Dropped from a bomber, the flying weapon is to hug the ground for long distances and zip through enemy air defenses to smash targets.

In describing the atomic plans, the Pentagon explicitly calls the cruise missile and related nuclear arms essential for “countering Russian aggression” in Eastern Europe.

The administration is also developing a hypersonic warhead that would zoom ahead of China’s rush to perfect its own. The U.S. version would be non-nuclear: The goal is a weapon so fast and precise that it relies on the raw force of impact to destroy a fixed target, such as a missile silo.

While that fulfills the president’s commitment to rely less on atomic weapons, it may prompt adversaries who cannot match the technology to depend more on nuclear arms.

Perry, the former defense secretary, argued that the diminished nuclear arms and the non-nuclear weapons that Obama is developing could make the unthinkable more likely.

“They make the weapons seem more usable,” he said, “even if there’s no credible plan for how you control escalation.”

Mark Gubrud, a nuclear weapons expert at the University of North Carolina, has lobbied for the negotiation of a global flight ban on the testing of hypersonic arms. If work continues, he wrote recently, the maneuverable warheads are likely to become a global reality in the next decade.

“The world has failed to put the nuclear genie back in the bottle,” Gubrud said. “And new genies are now getting loose.”