Sen. Mark Kelly, D-Ariz., raised sharp concerns Tuesday about the Golden Dome missile defense program, warning that Russia and China could perceive the system as a threat to their nuclear deterrents and respond by expanding their arsenals and deploying countermeasures.

Kelly opened his questioning by flagging what he called fundamental problems with the system's physics, particularly its space-based interceptor component, telling the panel the physics favor the offense.

He then pressed the panel on strategic stability, arguing that how adversaries view the system is as important as how the administration characterizes it.

Interceptors can be defeated by countermeasures. And often when we test these things, we don't account for those. But the other issue here is deterrence is about perception. And this administration might see Golden Dome as a defensive system. But what also matters here for strategic stability is how does China and Russia see this?

Mark Kelly at the hearing, saying the words quoted above. Captions from the auto-transcript.

Kelly invoked decades of U.S. nuclear strategy to frame the concern, linking offensive and defensive force levels.

For decades, U.S. nuclear strategy has recognized the relationship between strategic offensive and defensive forces. They're connected. The basic concern is pretty straightforward. If one side builds defenses that could substantially reduce the effectiveness of the other side's nuclear deterrent, the other side has an incentive to build more or better offensive weapons.

with more countermeasures. And that dynamic was part of the rationale behind negotiating limits on offensive and defensive systems during the Cold War. We considered this very seriously. So we need to understand what the response and expansion of U.S. strategic missile defense could generate.

Kelly asked Mr. Trachtenberg directly how Russia and China are currently assessing Golden Dome. Trachtenberg responded that both nations have already been modernizing their nuclear forces regardless of U.S. plans.

The hearing webcast at 43:59, as Mr. Trachtenberg spoke.
The hearing webcast at 43:59, as Mr. Trachtenberg spoke. U.S. Senate committee webcast (public domain)

I think Russia and China have been moving out irrespective of what the United States plans to do or may do in the future. As I said, both Russia and China have been modernizing their own nuclear systems, and I believe that they're doing that because their goals and objectives don't align with ours. I do think Russia and China will oppose Golden Dome.

Trachtenberg pushed back against the action-reaction framework Kelly raised, telling the senator there is no mechanistic formula linking U.S. missile defense to adversary buildups. Kelly responded by pressing him on what would be the logical response if a rival power believed the United States had built a system capable of defeating a large salvo of intercontinental ballistic missiles.

The exchange came during a hearing of the Senate Armed Services Subcommittee on Strategic Forces examining an update on the Golden Dome program. Kelly's questioning followed remarks by Sen. Mike Rounds, R-S.D., who argued that Golden Dome represented a necessary step toward preventing nuclear conflict and called for returning to a bipartisan approach to funding the program.