As North Korea dug tunnels at its nuclear test site last fall, 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.
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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 the modernization of weapons can produce a smaller and more reliable arsenal while making their use less likely because of the threat they can pose.
But critics, including a number of former Obama administration officials, say 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 strategists, 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.”
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. China is said to be especially worried about plans for a nuclear-tipped cruise missile.
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.
“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 Obama’s nuclear initiatives in total sees major progress toward the goals of a smaller force and a safer world.
Early in his tenure, Obama invested much political capital not in upgrades but reductions. In Prague in 2009, he pledged in a landmark speech that he would take concrete steps toward a nuclear-free world. The Nobel committee cited the pledge that year in awarding him the Peace Prize.
In 2010, the administration released a sweeping plan Obama called a fulfillment of his atomic vow. The overall plan was to rearrange old components of nuclear arms into revitalized weapons. The resulting hybrids would be far more reliable, meaning the nation would need fewer weapons in the far future.
Inside the administration, some early enthusiasts for Obama’s vision began to worry that it was being turned on its head.
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.
Some of the biggest names in nuclear strategy see a specific danger in the next weapon in the modernization lineup: the new cruise missile, a “standoff weapon” that bombers can launch far from their targets.
“Mr. President, kill the new cruise missile,” read the headline of a recent article by Weber, the former assistant secretary of defense, and William J. Perry, a secretary of defense under President Bill Clinton.
They argued that the cruise missile might sway a future president to contemplate “limited nuclear war.” Worse yet, they said, because the missile comes in nuclear and non-nuclear varieties, a foe under attack might assume the worst and overreact, initiating nuclear war.
The critique stung because Perry, now at Stanford, is a revered figure in Democratic defense circles and the mentor to Defense Secretary Ash Carter.
Other veterans of the Obama administration ask what happened.
“I think there’s a universal sense of frustration,” said Ellen O. Tauscher, a former undersecretary of state for arms control. She said many who joined the administration with high expectations for arms reductions now feel disillusioned.
“Somebody has to get serious,” she added.