Last summer in Berlin, President Obama called for "bold reductions" in U.S. and Russian tactical nuclear weapons to ease the risk of annihilation in Europe.
Obama was referring to the roughly 200 B61 nuclear bombs that the U.S. has deployed in five NATO nations stretching from the Netherlands to Turkey — as well as an even larger Russian arsenal estimated at 2,000 tactical weapons.
But since last summer, that hopeful outlook has evaporated. Russia's incursions into Ukraine and nuclear threats made by Russian President Vladimir Putin have killed any chance that the U.S. would withdraw its tactical nuclear weapons any time soon.
"Withdrawing our relatively few weapons would be the absolute wrong signal at this moment," said James Stavridis, the retired U.S. admiral who served as NATO chief until 2013 and is now dean of the Fletcher School of international affairs at Tufts University.
"Throughout my period of command as the NATO supreme allied commander, my personal view was that it was time to consider withdrawing the weapons from Europe," he said. "However, given Russian activities of the past months and the potential for a return to a period of significant friction between Russia and the alliance, I now believe we should keep the weapons in Europe, despite the costs and risks associated with doing so."
Support for nuclear deterrence has been echoing across Europe. Newer members of the North Atlantic Treaty Organization, including Poland and the Czech Republic, have openly advocated for continued deployment of U.S. nuclear weapons in Europe.
The maintenance of the B61 nuclear force on European soil involves trade-offs of cost, risk and deterrence.
The weapons spread over the continent are exposed to potential theft or accidents. But their presence is reassuring to some NATO allies, who believe the weapons show a strong U.S. commitment to their security. And proposed modifications to the B61 under an $8.1-billion Energy Department program should make them more accurate, enhancing their deterrence against Russia.
"As long as Russia is invading its neighbors, they are going to stay there at a minimum," said Stephen Rademaker, the assistant secretary of State for international security and nonproliferation during the George H.W. Bush administration.
Sleek and streamlined, packing an explosive force of up to 700 million pounds of TNT, the B61 thermonuclear weapon is the last of its kind, the only tactical nuclear bomb in the U.S. arsenal.
Unlike strategic weapons, designed to destroy cities and hardened military targets, the tactical weapons are intended for use on a battlefield, delivered by aircraft at treetop level or from high altitudes.
The bomb was designed in the 1960s during the Johnson administration, paralleling the technical breakthroughs of the space program. It was among the first compact nuclear weapons, measuring just 13 inches in diameter. The B61 comes in five models, one able to dial down its explosive power to just 2% of the bomb used in World War II on Hiroshima, according to outside estimates.
The U.S. began sending battlefield nuclear weapons to Europe in the 1950s, when it was feared the Soviet Union's conventional military superiority would allow it to overrun Western Europe.
All of those weapons, except the versatile B61, were long ago withdrawn.
Over the last 15 years, this U.S. nuclear umbrella has extended over an additional dozen Eastern European nations that gained entry into NATO.
As the Ukrainian crisis has unfolded, Obama has asserted that the alliance needs to "make concrete commitments to help Ukraine modernize and strengthen its security forces."
Pressure has built in Washington for the Obama administration to do more than increase the economic sanctions on Russia that seem to have had little effect.
Air Force Gen. Philip M. Breedlove, NATO's current supreme allied commander, told National Public Radio this month that Russia's actions were contrary to a future that allows European nations to choose their own destiny. "What's happened recently in Ukraine is what we thought would never happen again," he said.
The situation is unfolding at a critical juncture for the U.S. nuclear weapons complex.
Philip Coyle, who recently served as a security advisor in the Obama administration, said he believed the B61s should be withdrawn from Europe under a program that would allow for rapid deployment in the case of a military emergency.
"The more places you have them, the more opportunity there is for things to go wrong," said Coyle, formerly the deputy director of the Lawrence Livermore National Laboratory, the nuclear weapons design center in the Bay Area.
Hans Kristensen, a nuclear weapons expert at the Federation of American Scientists, has argued in recent days that the Ukraine crisis should not be a rationale for continued B61 deployments in Europe.
Their presence has not deterred Putin so far, and their future presence is merely an "echo from the past," he said.
But others say the B61's deployment in Europe is still a potent deterrent to the Cold War strategy that Putin has resurrected in the standoff with Ukraine.
Norton A. Schwartz, a retired four-star general and former Air Force chief of staff, said the aggressive Russian moves should reduce the uncertainty some NATO members have about the B61's relevance. "Does the NATO nuclear mission still provide some measure of deterrence, and with it, a degree of alliance cohesion?" he asked. "Experienced hands I know still think it does."
Linton Brooks, who negotiated the first strategic arms reduction treaty and later was the nuclear weapons chief in the Energy Department, said there would be wide support for a new treaty to eliminate the tactical weapons, but getting rid of them unilaterally would be a mistake.
"The danger is that it will be a signal to our Eastern European allies that we are lessening our support of them," he said. "Now is not the time."
But keeping the B61 in Europe will be part of an expensive nuclear modernization program. The bomb is the next major U.S. nuclear weapon to undergo a life extension program at an official cost of $8.1 billion, though the Pentagon estimates it would cost about $10 billion and outside groups say associated hardware would boost the total to $12 billion. The program would include upgrades to higher-yield B61s based in the U.S. as part of the strategic stockpile.
That cost increase is a reflection of the Energy Department's high-cost approach to its nuclear weapons mission, said Peter Stockton, a former Energy Department advisor and now senior investigator for the Project on Government Oversight.
Don Cook, chief of the U.S. nuclear weapons complex, said in a recent interview that the B61 is the oldest weapon in the U.S. inventory and that without a modernization program there would be some concerns about its future reliability.
Aging components, such as the conventional explosives that trigger the nuclear reactions and electronic arming systems, would be replaced. The bombs would get more formidable electronic and physical locks to thwart unauthorized use of a stolen weapon.
The modernization also would include a new tail fin assembly for greater accuracy and would allow a lower nuclear yield in attacking targets. "There would be less unintended collateral damage and less loss of life," Cook said.
Experts say those improvements would increase the weapon's deterrence value if the Russians believe the U.S. would be more likely to use the weapons because of their increased accuracy.
But such new capabilities are troubling to some experts who say the U.S. should not be thinking about how to make the weapons more useful.
"When the yield is lower and the system is more accurate, it makes them easier to use," Coyle said. "You don't want it to be tempting."