The United States has made an explosive accusation: China is carrying out secret nuclear explosive tests and preparing more. China says the charge is a lie. International monitoring officials say they have not seen confirming seismic evidence. But the dispute is not some academic argument between experts. It is a warning sign that one of the most important global taboos is cracking, and once that taboo breaks, it is hard to rebuild.

For decades, the world’s nuclear powers have mostly kept to a norm: no more nuclear explosions. If China is quietly pushing the limits while rapidly expanding its arsenal, it is exactly the kind of move a rising power would try, especially if it believes it can do it without getting caught. That makes this a real diplomatic crisis, and one that could become far more dangerous if it spirals into open nuclear testing by multiple countries.

A Public Accusation Made on Purpose

This was not a leak. It was an announcement.

Thomas DiNanno, the U.S. undersecretary of state for arms control and international security, disclosed the claim in a major international forum, the Conference on Disarmament in Geneva.

“I can reveal that the U.S. government is aware that China has conducted nuclear explosive tests, including preparing for tests with designated yields in the hundreds of tons,” DiNanno told delegates.

Then he pointed to a specific event. “China conducted one such yield producing nuclear test on June 22 of 2020,” he said.

DiNanno also argued that China’s military has been trying to conceal what it is doing because it knows these tests violate test-ban commitments.

What “Nuclear Testing” Means Now

When people hear “nuclear test,” they imagine a massive blast and a mushroom cloud. That is not what this dispute is mainly about.

The major nuclear powers have not detonated full-scale nuclear bombs in decades, but they have never stopped working on their arsenals. In the United States, weapons labs use computer simulations, science experiments, and underground explosive tests on weapon components. These are called “subcritical” tests because they do not create a nuclear chain reaction. They are designed to study materials and ensure reliability without crossing the line into a true nuclear detonation.

China and Russia are believed to run similar programs. The U.S. has long suspected they may also be doing experiments that cross into a small nuclear chain reaction, which is where the argument turns ugly.

Arms control expert Jeffrey Lewis summed up the core problem: “the Comprehensive Nuclear-Test-Ban Treaty does not define what an explosion is.” If the rules are fuzzy, a government can push into the gray zone and claim it is still obeying the spirit of a moratorium.

The biggest weakness in the U.S. claim is that the most obvious public indicator, seismic evidence, has not confirmed it.

Lewis reviewed publicly available seismic data for June 22, 2020, and said he found no signature of an explosive test near China’s test site. The Comprehensive Nuclear-Test-Ban Treaty Organization also said it had not seen seismic evidence of a test.

Rob Floyd, the CTBTO’s executive secretary, went further. He said the monitoring system “did not detect any event consistent with the characteristics of a nuclear weapon test explosion” on June 22, 2020, and that additional analysis did not change that conclusion.

The U.S. State Department has promised more disclosure later, telling NPR: “More information will be released soon.” A former senior U.S. official told CNN the information about the 2020 test had been declassified. But in public, the argument is still shaped by what outsiders can confirm, and right now outsiders cannot point to a clean seismic fingerprint.

That is why the accusation is simultaneously dramatic and destabilizing. It raises suspicion without settling the question.

DiNanno: China Is Hiding Tests

DiNanno is the senior U.S. official who put the claim on the record. He says China is hiding nuclear explosive tests using a technique called decoupling.

Decoupling is a way to reduce seismic signals by detonating inside a large underground cavity. Lewis explained it plainly: when the shock wave hits the cavity walls, much of the energy does not transfer into surrounding rock, and “the explosion looks much, much smaller than it really is.”

CNN described the same concept in practical terms: the cavern lessens the seismic activity from a nuclear explosion, making it harder to detect.

Decoupling is not magic. It does not erase a test, and global monitoring systems are extremely capable. But it can make a low-yield event harder to spot and harder to prove, which is exactly the point if a country wants the benefits of testing without the political blowback.

China’s Response: “Outright Lies”

China has rejected the accusation aggressively.

In a statement to AFP, China’s Foreign Ministry called the U.S. claims “outright lies.” The ministry said China “firmly opposes the US attempt to fabricate excuses for its own restarting of nuclear tests.”

China’s envoy, Shen Jian, dismissed the accusation as “false narratives” and accused the U.S. of fueling an arms race.

A spokesperson for the Chinese Embassy in Washington told CNN that China follows a “no first use” policy, focuses on self-defense, and adheres to its nuclear testing moratorium. The spokesperson said China is willing to work with other parties to uphold the CTBT’s authority, while urging the U.S. to take “concrete actions” to support disarmament and stability.

So the two sides are talking past each other. The U.S. says China is cheating. China says the U.S. is manufacturing a pretext.

More Than One Test

Even if the 2020 event remains disputed, DiNanno’s other claim is the one that should set off alarms: that China is preparing for tests with “designated yields in the hundreds of tons.”

That yield range matters because it sits between tiny experiments and full-scale detonations. It is the kind of number that suggests deliberate planning, not an accidental laboratory hiccup.

Ankit Panda of the Carnegie Endowment said there are “benign explanations and there are less benign explanations” for why China might designate yields like this.

The benign explanation is safety testing. Panda said such work can be used to ensure weapons are “one-point safe,” meaning that if one explosive component goes off, the weapon will not trigger a nuclear detonation. If a design fails, Panda warned it can “overrun” expected yield into the hundreds of tons, which can then force safer designs.

The less benign explanation is development of new weapons. Panda said China could be using gray-zone testing to refine new warhead designs. He and other experts point to China’s modernization push, including interest in warheads for advanced delivery systems. Lewis said the Pentagon believes China may be trying to develop “small” nuclear weapons with yields in the tens of kilotons.

This is where the strategic logic kicks in. A country building new warheads, new missiles, and new doctrine may feel pressure to validate designs. If it believes it can do that quietly, it will try.

Trump’s Push for a New Deal, and the Threat Behind It

This fight is happening right as the arms control system is weakening.

The last remaining U.S.–Russia treaty limiting deployed strategic nuclear weapons, New START, expired on February 5, 2026. DiNanno called that “the end of US unilateral restraint.”

President Trump has said he wants nuclear testing “on an equal basis” with other countries, including China. His administration has made clear it wants a broader nuclear weapons agreement that includes both China and Russia.

The message is pressure, not patience: if China will not join talks, the U.S. is signaling it may expand capability and reconsider its own testing posture.

Matthew Kroenig of the Atlantic Council argued that if China’s true concern is being outmatched, it should want arms control. “If they can get us to limit our weapons, that should be better for them,” he said.

But he believes China is refusing for a different reason. Beijing wants to build a true superpower nuclear force. Kroenig put it bluntly: “They’ve invested a lot in building this force. They didn’t spend all this money and bend all this metal to trade it away.”

That argument fits the bigger picture. China is still well behind the U.S. and Russia in total stockpile size, and joining limits too early could freeze it into permanent inferiority. That is why the U.S. believes pressure is necessary, and why China resists.

The world has been living through the longest period in history without a full-scale nuclear test. The last full-scale test was North Korea in 2017. The U.S. last tested in 1992. China last tested officially in 1996. Even without ratification of the Comprehensive Nuclear-Test-Ban Treaty by the U.S. and China, the voluntary moratoria have helped preserve a fragile norm.

That norm is now under strain.

Daryl Kimball of the Arms Control Association warned that “if there is any true violation of the test ban treaty, that’s a big problem, but simply complaining about it doesn’t solve the problem.” And Jeffrey Lewis warned that once countries start testing again, the process escalates quickly: “There’s no real stopping point once we all start doing bigger and bigger tests.” He said the end result could look “just like the Cold War.”

This is why the accusation matters even without public proof. It injects mistrust at the exact moment treaty limits are fading. If the U.S. and China treat nuclear testing as a bargaining chip, the world could stumble into a new arms race driven by suspicion, pride, and the belief that the other side is cheating.

China is expanding. The U.S. is reacting. Each side believes its position is defensive. That is how dangerous cycles begin.