By William J. Broad and David E. Sanger, New York Times

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 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.

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 itself for speeding ahead with a nuclear “modernization” that, in the name of improving safety and reliability, risks throwing fuel on the fire.

President Barack 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.”

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 R. 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.

Moscow is fielding big missiles topped by miniaturized warheads, and experts fear that 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 hardly 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 M. 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 like 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?

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

Obama, speaking at the summit meeting’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.”

White House officials say they try to tamp down any worried reactions to the new developments. In an interview, Avril Haines, deputy national security adviser, said, “When tensions develop, we take steps to avoid unnecessarily raising the temperature.”

Obama came to office in 2009 eager to “reset” relations with Moscow, reduce America’s reliance on nuclear arms and move toward their elimination. He was the first president to make nuclear disarmament a centerpiece of U.S. defense policy.

Russia initially cooperated, signing in 2010 the New START treaty, which made modest reductions in strategic nuclear forces.

That year, Obama offered another olive branch: He ordered the U.S. military to reduce the number of warheads atop its land-based missiles to one, from as many as three. That was a signal to show the missiles were more about defense than offense.

Moscow did not reciprocate. Instead, with treaty ink barely dry, it began deploying a new generation of long-range missiles that bore four miniaturized warheads. It continues such actions, even while adhering to overall treaty limits.

At this month’s summit meeting, Obama blamed Putin’s return to the Russian presidency in 2012 for preventing further arms reductions, saying the Kremlin was “emphasizing military might over development.”

William J. Perry, defense secretary under President Bill Clinton and one of the most influential nuclear experts in the Democratic Party, said he worried that Moscow 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 White House 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 Beijing’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.”

No major nuclear power is more threatened by the U.S. advances than China, analysts say. A pre-emptive strike, they note, might easily do in its relatively small arsenal.

For a decade, Christopher P. Twomey, a national security expert at the Naval Postgraduate School, in Monterey, California, has helped run informal meetings between U.S. and Chinese analysts, government officials and military officers.

Last year in testimony to the U.S.-China Economic and Security Review Commission, created by Congress, he reported that Beijing felt increasingly encircled. It sees Washington’s hypersonic glider as a way to attack China without crossing the nuclear threshold, complicating its assessment of nuclear retaliation.

Twomey said Chinese leaders had similar apprehensions about growing numbers of anti-missile interceptors on U.S. warships in the Pacific as well as bases in California and Alaska.

Finally, he added, Beijing views Washington’s nuclear modernization “with much trepidation.” Specifically, he cited plans for a new guided bomb and the advanced cruise missile, as well as new delivery systems.

“Beijing has responded to these changes,” Twomey testified, “and will likely continue to do so over the next decade.”

China has re-engineered many of its long-range missiles to carry multiple warheads, rejecting Obama’s example — and following Putin’s. The step troubled analysts because Beijing for decades has known how to miniaturize warheads and put two or more atop a single missile.

It turns out that Beijing is discussing an even more ominous step.

For decades, Washington and Moscow have kept their nuclear forces on high alert so that, in theory, military authorities can fire missiles if networks of radars, satellites and computers detect an incoming strike. The tactic is meant to dodge a crippling blow that might curb or eliminate a nation’s ability to retaliate.

Critics see the “launch on warning” tactic as greatly increasing the risk of accidental war. In the past, they note, false alerts have repeatedly brought the world to the brink of disaster.

Last year, the Chinese military declared in an official document that the nation seeks to “improve strategic early warning” for its nuclear forces.

Early this year, the Union of Concerned Scientists, a private group in Cambridge, Massachusetts, that backs arms control, published a report on the intensifying launch-on-warning debate. It said the Obama administration’s arms modernizations “are the most prominent external factor influencing Chinese advocates.”

Advocates of arms control say their field needs reinvention. They see the counting of warheads and delivery vehicles — the traditional levers — as unsuitable for arresting the development of the new weapons.

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.”

To Learn More:

Nations Defy Nuclear Treaty by Expanding Weapons Programs (by Noel Brinkerhoff, AllGov)

Obama Pledges $11 Billion to Upgrade U.S. Nuclear Weapons (by Noel Brinkerhoff, AllGov)    

Americans Willing to Use Nuclear Weapons (by Noel Brinkerhoff, AllGov)    

Nobel Peace Prize Winner Obama Increases Spending for Nuclear Weapons (by Noel Brinkerhoff, AllGov)