In the wake of September 11, 2001, and last week's terrorist bombings in Riyadh, America's relationship with Saudi Arabia has sunk to new lows. On the right, neoconservatives rail against Saudi support for extremist Islam and fume over the kingdom's refusal to allow American airmen to fly sorties from Prince Sultan Air Base during the Afghan war. The conservative magazine Commentary recently ran an article titled "Our Enemies, the Saudis," while The Wall Street Journal's editorial page has blasted the Saudis for funneling money to militant Islamists. For their part, liberals point to Saudi Arabia's lack of political freedom as evidence that America's realpolitik-based Middle East policy is morally corrupt: This month, The Washington Post editorial page took the Bush administration to task for not citing Saudi Arabia as a violator of human rights and religious freedoms. In fact, hawks and doves who disagree about virtually everything else agree that the United States would be better off without its Saudi "ally." "I think we're heading for a divorce," Youssef Ibrahim, former head of a Council on Foreign Relations task force on U.S.-Saudi relations, told BusinessWeek last year.

Realists counter that the United States needs Saudi oil and Saudi military bases. But there's a less obvious argument for making sure the long-standing Washington-Riyadh partnership doesn't fracture: If it does, the Saudis might well go nuclear.

Saudi Arabia could develop a nuclear arsenal relatively quickly. In the late '80s, Riyadh secretly purchased between 50 and 60 CSS-2 missiles from China. The missiles were advanced, each with a range of up to 3,500 kilometers and a payload capacity of up to 2,500 kilograms. What concerned observers, though, was not so much these impressive capabilities but rather the missiles' dismal accuracy. Mated to a conventional warhead, with a destructive radius of at most tens of meters, these CSS-2 missiles would be useless-their explosives would miss the target. But the CSS-2 is perfect for delivering a nuclear weapon. The missile itself may miss by a couple of kilometers, but, if the bomb's destructive radius is roughly as large, it will still destroy the target. The CSS-2 purchase, analysts reasoned, was an indication that the Saudis were at least hedging in the nuclear direction.

July 1994 brought more news of Saudi interest in nuclear weapons when defector Mohammed Al Khilewi, a former diplomat in the Saudi U.N. mission, told London's Sunday Times that, between 1985 and 1990, Saudi Arabia had actively aided Iraq's nuclear weapons program, both financially and technologically, in return for a share of the program's product. Though Khilewi produced letters supporting his claim, no one has publicly corroborated his accusations. Still, the episode was unsettling. Then, in July 1999, The New York Times reported that Saudi Defense Minister Prince Sultan bin Abdulaziz Al Saud had recently visited sensitive Pakistani nuclear weapons sites. Prince Sultan toured the Kahuta facility where Pakistan produced enriched uranium for nuclear bombs-and which, at the same time, was allegedly supplying materiel and expertise to the North Korean nuclear program. The Saudis refused to explain the prince's visit.

If Saudi Arabia chose the nuclear path, it would most likely exploit this Pakistani connection. Alternatively, it could go to North Korea or even to China, which has sold the Saudis missiles in the past. Most likely, as Richard L. Russell, a Saudi specialist at National Defense University, argued two years ago in the journal Survival, the Saudis would attempt to purchase complete warheads rather than build an extensive weapons-production infrastructure. Saudi Arabia saw Israel destroy Iraq's Osirak reactor in 1981, and it is familiar with America's 1994 threat to bomb North Korea's reactor and reprocessing facility at Yongbyon. As a result, it would probably conclude that any large nuclear infrastructure might be preemptively destroyed. At the same time, Riyadh probably realizes that America's current hesitation to attack North Korea stems at least in part from the fact that North Korea likely already has one or two complete warheads, which American forces would have no hope of destroying in a precision strike. By buying ready-made warheads, Riyadh would make a preemptive attack less likely. And, unlike recent proliferators such as North Korea, the Saudis have the money to do so.

Some analysts would argue that Saudi Arabia could enhance its security by undertaking a conventional, rather than a nuclear, buildup. But, as Middle East expert F. Gregory Gause III argues in a new Brookings Institution paper, Saudi Arabia's military is "weak in part by design, to prevent the internal threat of a military coup." Until the Saudi government is widely legitimate in the eyes of its own people-and that day seems a long way off-it is unlikely to build a large conventional military. By contrast, Saudi leaders might favor a nuclear arsenal, believing it enhances their security against external threats while being fairly useless to coup-plotters.

Why would Riyadh want nukes now? Because of a potentially dangerous confluence of events. The rapidly progressing nuclear program of traditional rival Iran has no doubt spooked the Saudi leadership. Last fall, dissidents revealed the existence of a covert Iranian uranium-enrichment program, forcing analysts to drastically revise down their estimates of how long it might take Iran to obtain nuclear weapons. Reacting to that development, Patrick Clawson, deputy director of the Washington Institute for Near East Policy, recently wrote that "Saudi Arabia is the state most likely to proliferate in response to an Iranian nuclear threat" because, he argued, the Saudis fear a nuclear-armed Iran could have designs on Saudi Arabia, a Sunni monarchy that is home to a large number of oppressed Shia. After all, Tehran has for years allegedly supported Shia terrorist groups operating in Saudi Arabia and was blamed by many analysts for the 1996 Khobar Towers bombing.

Holding back the Saudi nuclear program, of course, has been the kingdom's relationship with the United States. Though America has never signed a formal treaty with Riyadh, since World War II the United States has made clear by its actions-most notably, by protecting Saudi Arabia during the 1991 Gulf war-and by informal guarantees given to Saudi leaders by American officials that it will protect the monarchy from outside threats.

Since the September 11 attacks, though, that relationship has grown increasingly frail. When a RAND analyst last summer told the Defense Policy Board, then chaired by Richard Perle, that Saudi Arabia was "the kernel of evil, the prime mover, the most dangerous opponent" in the Middle East, he not only raised hackles in Riyadh, he reflected the opinion of many close to the Bush administration. R. James Woolsey, former CIA director and White House confidant, was even more emphatic in a speech last November, referring to "the barbarics [sic], the Saudi royal family." The recent decision by Washington to pull most of its forces out of Saudi Arabia, reducing its deployment from 5,000 to 400 personnel and moving its operations to Qatar, has added facts on the ground to the rhetorical barrage. This recent decline in U.S.-Saudi relations can hardly make the Saudi royal family feel secure.

Suddenly removing the U.S. security blanket just as regional rivalries are intensifying could push the Saudis into the nuclear club. That's a scary prospect, particularly when you consider the possibility of Islamists overthrowing the monarchy. Instead, the United States should be careful to maintain Saudi Arabia's confidence even as the two nations inevitably drift apart. The United States might even extend an explicit security guarantee to the Saudis, the kind of formal treaty it gave Europe to keep it non-nuclear during the cold war-and the kind of formal arrangement Washington and Riyadh have never signed before. Such a formal deal could raise anti-American sentiment in the desert kingdom. But the alternative might be worse.