As North Korea dug tunnels at its nuclear test site last fall, watched by U.S. spy satellites, the Obama administration was preparing a test of its own in the Nevada desert.
A fighter jet took off with a mock version of the nation's first precision-guided atom bomb. Adapted from an older weapon, it was designed with problems like North Korea in mind: Its computer brain and four maneuverable fins let it zero in on deeply buried targets like testing tunnels and weapon sites. And its yield, the bomb's explosive force, can be dialed up or down depending on the target, to minimize collateral damage.
In short, while the North Koreans have been thinking big — claiming to have built a hydrogen bomb, a boast that experts dismiss as wildly exaggerated — the Energy Department and the Pentagon have been readying a line of weapons that heads in the opposite direction.
The build-it-smaller approach has set off a philosophical clash among those in Washington who think about the unthinkable.
President Barack Obama has long advocated a "nuclear-free world." His lieutenants argue that modernizing existing weapons can produce a smaller and more reliable arsenal while making their use less likely because of the threat they can pose. The changes, they say, are improvements rather than wholesale redesigns, fulfilling the president's pledge to make no new nuclear arms.
But critics, including a number of former Obama administration officials, look at the same set of facts and see a very different future. The explosive innards of the revitalized weapons may not be entirely new, they argue, but the smaller yields and better targeting can make the arms more tempting to use — even to use first, rather than in retaliation.
Gen. James E. Cartwright, a retired vice chairman of the Joint Chiefs of Staff who was among Obama's most influential nuclear strategist, said he backed the upgrades because precise targeting allowed the United States to hold fewer weapons. But "what going smaller does," he acknowledged, "is to make the weapon more thinkable."
As Obama enters his final year in office, the debate has deep implications for military strategy, federal spending and his legacy.
The B61 Model 12, the bomb flight-tested last year in Nevada, is the first of five new warhead types planned as part of an atomic revitalization estimated to cost up to $1 trillion over three decades. As a family, the weapons and their delivery systems move toward the small, the stealthy and the precise.
Already there are hints of a new arms race. Russia called the B61 tests "irresponsible" and "openly provocative." China is said to be especially worried about plans for a nuclear-tipped cruise missile. And North Korea last week defended its pursuit of a hydrogen bomb by describing the "ever-growing nuclear threat" from the United States.
The more immediate problem for the White House is that many of its alumni have raised questions about the modernization push and missed opportunities for arms control.
"It's unaffordable and unneeded," said Andrew C. Weber, a former assistant secretary of defense and former director of the Nuclear Weapons Council, an interagency body that oversees the nation's arsenal. He cited in particular the advanced cruise missile, estimated to cost up to $30 billion for roughly 1,000 weapons.
"The president has an opportunity to set the stage for a global ban on nuclear cruise missiles," Weber said in an interview. "It's a big deal in terms of reducing the risks of nuclear war."
Last week, Brian P. McKeon, the principal deputy undersecretary of defense for policy, argued that anyone who looks impartially at Obama's nuclear initiatives in total sees major progress toward the goals of a smaller force and a safer world — themes the White House highlighted Monday in advance of the president's State of the Union address.
"We've cleaned up loose nuclear material around the globe, and gotten the Iran deal," removing a potential threat for at least a decade, McKeon said. He acknowledged that other pledges — including treaties on nuclear testing and the production of bomb fuel — have been stuck, and that the president's hopes of winning further arms cuts in negotiations with Russia "ran into a blockade after the events in Ukraine."
He specifically defended the arsenal's modernization, saying the new B61 bomb "creates more strategic stability."
In late 2013, the first of the former insiders spoke out. Philip E. Coyle III and Steve Fetter, who had recently left national security posts, helped write an 80-page critique of the nuclear plan by the Union of Concerned Scientists, a private group that made its name during the Cold War, arguing for arms reductions.
U.S. allies and adversaries, the report warned, may see the modernization "as violating the administration's pledge not to develop or deploy" new warheads. The report, which urged a more cautious approach, cited a finding by federal advisory scientists: that simply refurbishing weapons in their existing configurations could keep them in service for decades.
"I'm not a pacifist," Coyle, a former head of Pentagon weapons testing, said in an interview. But the administration, he argued, was planning for too big an arsenal. "They got the math wrong in terms of how many weapons we need, how many varieties we need and whether we need a surge capacity" for the crash production of nuclear arms.
The insider critiques soon focused on individual weapons, starting with the B61 Model 12. The administration's plan was to merge four old B61 models into a single version that greatly reduced their range of destructive power. It would have a "dial-a-yield" feature whose lowest setting was only 2 percent as powerful as the bomb dropped on Hiroshima in 1945.
The plan seemed reasonable, critics said, until attention fell on the bomb's new tail section and steerable fins. The Federation of American Scientists, a Washington research group, argued that the high accuracy and low destructive settings meant military commanders might press to use the bomb in an attack, knowing the radioactive fallout and collateral damage would be limited.
Last year, Cartwright echoed that point on PBS' "NewsHour." He has huge credibility in nuclear circles: He was head of the U.S. Strategic Command, which has military authority over the nation's nuclear arms, before serving as vice chairman of the Joint Chiefs of Staff.
In a recent interview in his office at the Center for Strategic and International Studies, in Washington, Cartwright said the overall modernization plan might change how military commanders looked at the risks of using nuclear weapons.
"What if I bring real precision to these weapons?" he asked. "Does it make them more usable? It could be."
In an interview, James N. Miller, who helped develop the modernization plan before leaving his post as undersecretary of defense for policy in 2014, said the smaller, more precise weapons would maintain the nation's nuclear deterrent while reducing risks for civilians near foreign military targets.
"Though not everyone agrees, I think it's the right way to proceed," Miller said. "Minimizing civilian casualties if deterrence fails is both a more credible and a more ethical approach."
Cartwright summarized the logic of enhanced deterrence with a gun metaphor: "It makes the trigger easier to pull but makes the need to pull the trigger less likely."
Administration officials often stress the modernization plan's benign aspects. Facing concerned allies, Madelyn R. Creedon, an Energy Department deputy administrator, argued in October that the efforts "are not providing any new military capabilities" but simply replacing wires, batteries, plastics and other failing materials.
"What we are doing," she said, "is just taking these old systems, replacing their parts and making sure that they can survive."