Matthew Bunn is the Schlesinger professor of the practice of energy, national security and foreign policy at the Harvard Kennedy School. He has helped lead the Project on Managing the Atom at the Belfer Center for Science and International Affairs for more than two decades, and previously he worked in the White House Office of Science and Technology Policy from 1994-1996.
In this interview, he discusses the end of New START, the last treaty limiting U.S. and Russian nuclear forces. Signed in 2010, it expires on February 5. Bunn also wrote about the looming deadline in a recent column for The Hill.
1. What changes the day after New START expires?
For the first time in half a century, we will be living in a world with no agreed limits on the United States and Russia building up their nuclear forces. For decades, nuclear arms control has improved U.S., Russian, and global security and reduced the risk of nuclear war by reducing tensions, increasing predictability and transparency, and limiting nuclear forces of particular concern. The mere act of being able to discuss and reach agreements on limiting the most fearsome forces on each side tends to reduce perceptions that the other side is implacably bent on your destruction.
2. In your recent piece in The Hill , you argue that Putin’s offer to stay within New START limits for a year and begin new discussions is “not a great offer” but still one worth taking. Why have the U.S. and Russia waited until New START’s expiration to begin these talks?
Unfortunately, hostile relations and the ups and downs of politics have prevented serious discussions. A few years after New START entered into force, U.S.-Russian political relations soured when Russia seized the Crimean peninsula in Ukraine and destabilized eastern Ukraine. (Russia had its own long-standing complaints about U.S. behavior.) President Trump, when he took office, did not want to extend an Obama-era treaty. He wanted to do a new deal but didn’t get started until very late, and talks never went far. In 2021, President Biden and President Putin agreed to extend the treaty for five years and to start talks on strategic stability—but after one round, Russia launched its full-scale invasion of Ukraine, and talks collapsed. Then Russia, complaining about Ukrainian strikes into Russia, refused to restart the strategic stability talks and cut off the on-site inspections and data exchanges called for by the treaty. It was only in September 2025 that Putin offered to stay within the treaty limits for now and restart talks on what might come next.
3. If a “strategic pause,” as you say, would buy time—what should the U.S. do during that pause to avoid simply drifting into a new arms buildup?
There needs to be a serious exploration of options for managing nuclear competition in the much more complex security situation that now exists in the world. China has launched a major nuclear buildup, and within a few years will have something like 1,000 nuclear weapons. Evolving technologies also complicate the picture, including missile defenses such as President Trump’s proposed “Golden Dome,” precision conventional weapons that can carry out strategic missions, artificial intelligence, cyber, space and counter-space, and more. The challenges are great, so both time and creativity will be needed to find paths forward—which probably will not be one big treaty, but rather a variety of initiatives and accords covering different issues.
4. For decades, arms control meant Washington and Moscow. Now, as you suggest in The Hill, China is in the mix, and it currently has “no interest in discussing limits on its nuclear buildup.” Do you believe China will be more amenable to a treaty in the coming years?
China is certainly not ready today. But they need to understand that continuing their buildup with no limits or predictability at all will provoke a U.S. buildup in response, and possibly a collapse of U.S.-Russian arms control. That will not be in China’s national interest. That’s why I’ve been working to make the case to Chinese colleagues that some form of restraint—and there are many different possibilities—is in China’s own interest. States agree to limit their forces when they think the [reciprocal] limits on their adversary’s forces are sufficient to their overall security interest. The United States should be willing to make some offers that might convince China and Russia to negotiate seriously on future nuclear restraint. Ultimately, an unconstrained three-way nuclear arms competition would leave all sides poorer and less secure.
5. If an unconstrained nuclear buildup were to begin, how hard would it be to reverse or slow down?
More than numbers [of nuclear warheads], the most intense areas of competition now have to do with technology—making different types of delivery systems that are more accurate and harder to defend against and integrating nuclear forces with the full suite of evolving non-nuclear capabilities.
Nuclear weapons programs, once launched, have a substantial momentum of their own, and are hard to stop and reverse. A key advantage of continuing some form of nuclear restraint is predictability. If Washington, Moscow, and Beijing each plan their forces around their worst-case visions of what the other countries’ forces might look like in 20 years’ time, we’ll be headed for an intense three-way competition that will be hard to stop. But if agreed limits set the parameters of what they need to plan for, competition can be kept with safer and saner bounds.
