Iran tried to buy three nuclear weapons from Pakistan at the end of the 1980s. Islamabad rebuffed the attempt but ended up transferring to Tehran bomb blueprints, centrifuge parts, and a list of black market suppliers of components. So says Dr. Abdul Qadeer Khan, the father of Pakistan’s atomic bomb, in an official account revealed on Sunday.

Khan’s statements undermine both Iranian denials of nuclear weapon ambitions and Islamabad’s assertions that it had no hand in proliferating nuclear weapons technology. Up to now, Pakistan’s government has maintained that Dr. Khan, who confessed to selling bomb know-how to various countries, went rogue, acting without official sanction. Khan’s account of events, although self-serving, appears generally credible because it is consistent with known facts.

Now, Khan’s black market ring has been dismantled. No one is thinking of punishing Islamabad for nuclear proliferation. Iran, on the other hand, faces the possibility of even more punitive measures for its suspected nuclear weapons program. The P5+1, the five permanent members of the Security Council and Germany, is moving to impose a fourth set of sanctions on the Islamic Republic for its failure to comply with earlier U.N. demands.

The only thing that stands between Iran and new sanctions is veto-wielding China. In recent months Moscow has distanced itself from the “atomic ayatollahs,” and this has left Beijing as their primary backer.

The significance of Khan’s assertions is that they undermine the stout Chinese defense of Iran. First, they highlight long-held Iranian ambitions to build an atomic arsenal.

Second, by detailing how the Pakistani government was involved in nuclear transfers to Iran, Khan raises new questions about Beijing’s role. Why? The Pakistani nuclear weapons program is essentially an extension of the Chinese one. China, beginning around 1974, transferred bomb technology to Pakistan. Beijing’s assistance was crucial, extensive, and continuous. As Gary Milhollin of the Wisconsin Project on Nuclear Arms Control has noted, “If you subtract China’s help from Pakistan’s nuclear program, there is no nuclear program.” Moreover, Beijing has remained involved in Islamabad’s nuclear efforts, long after the events Khan so meticulously describes in Sunday’s statements.

The continuation of Chinese involvement in the Pakistani program was revealed when Islamabad ended the Khan ring. Due to Chinese pressure, Pervez Musharraf, then the country’s strongman leader, conducted a hurried probe, forced Khan’s confession, and then immediately pardoned him in 2004 to cut off any disclosures embarrassing to Beijing, which supported the controversial decision to end the inquiry prematurely. Given China’s role in the Pakistani nuclear program and its influence in Islamabad, it was not possible for Khan, with official blessing, to transfer Chinese technology to Iran without Beijing’s knowledge and consent.

Dr. Khan apparently did not mention China’s involvement in the statements disclosed Sunday, but the revelation of official Pakistani links to proliferant activities puts Beijing on the spot nonetheless. As time goes on, we are finding more facts linking China to Iran’s efforts to build the most destructive weaponry in history, including direct transfers of equipment and technology to Iran. Much, if not most, of this information about Chinese involvement remains classified in Washington, however.

Why are we helping China keep its secrets? Perhaps the Obama administration should start disclosing—or start threatening to disclose—what else we know about Beijing’s support for the mullahs.

Gordon G. Chang is the author of "The Coming Collapse of China." He writes a weekly column at Forbes.com.

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Gordon G. Chang is the author of "The Coming Collapse of China." He writes a weekly column at Forbes.com. Follow him on Twitter @GordonGChang.