It’s a pivotal moment for nuclear weapons.
Tensions between the United States and Russia, the two major nuclear powers, are high. Longstanding nuclear pacts have crumbled. China, which trails well behind the United States and Russia in nuclear arms, is beefing up its arsenal. North Korea is a nuclear threat, and Iran is on the threshold.
Meanwhile, the United States is in the midst of an extensive update of its nuclear weapons. And when Donald Trump takes office as president on January 20, he will, for a second time, be responsible for those weapons. Here are some of the big issues we’ll be watching.
What will happen to nuclear arms control?
During the Cold War, in 1986, the world’s nuclear stockpiles swelled to more than 70,000 nuclear warheads. Now there are around 12,000. That reduction is largely thanks to a series of treaties between the United States and Russia (or previously the Soviet Union), the two nations that maintain the bulk of the weapons.
But now, the only remaining nuclear arms control treaty between the United States and Russia, the New Strategic Arms Reduction Treaty, or New START, is set to expire in February 2026. “If that treaty expires without a replacement, it will be the first time in 50 years that we do not have any agreed restrictions on nuclear arsenals with Russia,” says Steve Fetter, a professor of public policy at the University of Maryland in College Park.
Trump has a track record of pulling out of arms control deals. He withdrew the United States from the Iran nuclear deal in 2018, from the Intermediate-Range Nuclear Forces Treaty in 2019, and from the Treaty on Open Skies in 2020. In Trump’s previous term, New START came close to expiring before President Joe Biden renewed it shortly after assuming office in 2021.
But Trump, who touts himself as a dealmaker, could instead pull off an agreement, Fetter says, pointing to Trump’s efforts to negotiate with North Korea during his previous term. “I think that suggests that it’s possible.”
Deal or not, using just a fraction of the world’s nuclear weapons could cause global devastation. In the 1980s, scientists predicted that smoke from fires caused by the blasts would be lofted into the stratosphere, where it would block sunlight, causing global cooling and thereby inducing widespread famine. Observations of wildfire smoke recently strengthened the case that such a war could cause a “nuclear winter,” scientists reported in 2019 in Science.
What will happen to the U.S. nuclear arsenal?
In the meantime, the United States is neck-deep in a sweeping effort to modernize its nuclear arsenal, replacing or revamping nearly every component, including the nuclear warheads themselves and the missiles, submarines and bombers that carry them.
The decades-long affair is already over budget and behind schedule, and experts say there’s little prospect of Trump significantly tinkering with that process. “His ability to change what’s actually happening on the ground in the U.S. is pretty close to zero,” says Lisbeth Gronlund, a researcher in nuclear security and policy at MIT.
One avenue where Trump could make an impact is the proposed development of a new type of weapon, a nuclear-armed sea-launched cruise missile. Currently, the United States has 14 submarines that carry nuclear-armed ballistic missiles, so named because they fly in a ballistic trajectory: After launch they fall back to Earth in an arcing path determined by gravity. Cruise missiles, on the other hand, undergo powered flight for the duration of their travel.
The fuzzy calculus of nuclear weapons involves ensuring that nuclear-armed countries are deterred from attacking one another based on fear of retaliation. Arguing that the addition of the new missile would diversify the U.S. nuclear arsenal and thus enhance that deterrent, the first Trump administration proposed the weapon in 2018. But the Biden administration wasn’t keen on the idea and tried to cancel its development. Trump could rekindle the effort, and that could ruffle feathers. “This is a totally new kind of weapon. It’s an aggressive move, and both China and Russia will perceive it as such,” Gronlund says.
Another issue is how many arms the U.S. has deployed. If New START dissolves, Trump will have more freedom to boost those numbers. “That’s something I will be looking for,” Gronlund says. “Is he going to be calling for the U.S. deploying more weapons?”
Each new administration performs a nuclear posture review, which lays out priorities and strategies. The Trump team is likely to do so in 2026. “That will give a very good blueprint for how the administration is viewing the role of nuclear weapons and how it will approach investing in the kind of force that they would like to see,” says Sharon Squassoni, a research professor of international affairs at George Washington University in Washington, D.C.
Could the United States return to nuclear weapons testing?
Some experts also speculate that Trump could break the longstanding moratorium on testing nuclear weapons. This would be a seismic shift. The United States hasn’t performed any explosive nuclear tests since 1992. The only country to test a nuclear weapon this century is North Korea. But some in the Trump orbit have called for a return to testing, including a former national security advisor to Trump. And officials in the previous Trump administration discussed the possibility of resuming tests, according to a report in the Washington Post.
Coupled with Trump’s willingness to challenge norms, that suggests a resumption of tests is possible, some experts say. “He just threatened tariffs on Denmark if they don’t give us Greenland. In that world, is a return to nuclear testing credible?” Squassoni asks. “Yeah, sure.”
In 1996, the United States signed the Comprehensive Nuclear-Test-Ban Treaty, which prohibits nuclear weapons tests around the globe. Although the treaty was never ratified by Congress, the U.S. has abided by the treaty’s provisions. Prior to the treaty, more than 2,000 nuclear tests caused untold damage to human health and the environment.
Although underground tests reduce the dispersal of radioactive fallout compared to atmospheric tests, they don’t assure the bombs’ impact is contained. Radioactive material can accidentally be released into the atmosphere or migrate via groundwater, researchers reported in 2024 in Bulletin of the Atomic Scientists.
The United States resuming tests may encourage other countries with nuclear weapons to follow suit, experts say, and could ignite backlash from non-nuclear weapons countries.
In lieu of explosive tests, the United States currently performs nonexplosive “subcritical” nuclear tests. The Nevada National Security Site, northwest of Las Vegas, will soon host an enormous machine called Scorpius, with a particle accelerator the length of a football field designed to generate X-ray images of experiments with plutonium.
Such efforts, combined with extensive computer simulations of nuclear weapons, have made explosive tests obsolete in the eyes of many. “I don’t think it’s necessary,” Fetter says. “I think it’d be a huge mistake. But it’s something that I and others will be looking at.”