When Iran defiantly cut the locks and seals on its nuclear enrichment plants in January and restarted its effort to manufacture atomic fuel, it forced the world to confront a momentous question: How long will it be before Tehran has the ability to produce a bomb that would alter the balance of power in the Middle East?
Iran's claims that it is racing forward with enrichment have created an air of crisis as the board of the International Atomic Energy Agency prepares to meet tomorrow in Vienna, Austria, before the U.N. Security Council takes up the Iran file for possible penalties.
Yet behind the sense of immediate alarm lies a more complex picture of Iran's nuclear potential. Interviews with many of the world's leading nuclear analysts and a review of technical assessments show that Iran continues to wrestle with problems that have slowed its nuclear ambitions for more than two decades.
Obstacles, the experts say, remain at virtually every step on the atomic road. And the most significant, they say, involve the two most technically challenging aspects of the process - converting uranium ore to a toxic gas and, especially, spinning that gas into enriched atomic fuel.
According to analysts, the Iranians need to do repairs and build new machines at a prototype plant before they can begin enriching even modest quantities of uranium.
And then, for a decade, they would have to mass produce 100 centrifuges a week to fill the cavernous industrial enrichment halls at Natanz. What is more, the gas meant to feed those machines is laced with impurities.
The perception gap was underscored in February when Tehran issued a stark warning. By late this year, Iranian officials said, they would begin installing nearly 3,000 centrifuges at the giant Natanz plant, buried deep underground. That many centrifuges, international inspectors knew, could make fuel for up to 10 nuclear warheads every year.
In Washington and Europe, the announcement was dismissed as an empty boast. "Maybe they can move that fast," said a senior U.S. official who tracks Iran's program but who declined to be named. "But they would need lots of help, luck and prayer."
Tehran maintains that it has every right to master the atomic basics in pursuit of a peaceful program of nuclear power. But more and more countries have come to view that as a cover story.
Estimates of just when Iran might acquire a nuclear weapon range from alarmist views of only a few months to roughly 15 years. American intelligence agencies say it will take five to 10 years for Iran to manufacture the fuel for its first atomic bomb. Most forecasters acknowledge that secret Iranian advances or black market purchases could produce a technological surprise.
Conservative forecasts often take into account not only the technical difficulties but also a political judgment: that Tehran will run for the finish line - making its first bomb - only when it can rapidly produce a large arsenal.
Some of Iran's nuclear troubles can be traced to wavering political commitment by mullahs more interested in creating a theocracy than unlocking the secrets of the atom. And many top scientists fled after the Islamic revolution of 1979.
But the United States created other obstacles. In the 1990s, it pressured Russia, China and other nations to end deals that would have given the Iranian program a jump-start. Some of those maneuvers were covert; some played out in the press.
"In retrospect, we impeded a lot more of their progress than we knew," said Robert J. Einhorn, a central player in nuclear diplomacy in the Clinton administration and the early days of the Bush administration.
By all accounts, Iran's oldest and most daunting development problem involves centrifuges - temperamental machines whose rotors can spin extraordinarily fast to enrich uranium. After two decades of effort, Iran is barely out of the starting gate.
Tehran began in 1987 to buy drawings and centrifuge parts from Pakistani Abdul Qadeer Khan, then moved on to Moscow.
But after the Clinton administration persuaded Moscow to back out, Iran accelerated its secret drive to copy Khan's centrifuges. It also started building the huge enrichment plant near Natanz. The pilot factory there was to house 1,000 centrifuges; the main plant would shelter 50,000 machines underground.
In August 2002, Iranian dissidents revealed the existence of the Natanz site, beginning the current confrontation with the West. The next year, Iran agreed to suspend work while negotiating with Europe over the program's fate.
But when operators shut down an experimental cascade of 164 centrifuges at Natanz, about 50 of them broke or crashed, according to a January report by David Albright and Corey Hinderstein of the Institute for Science and International Security, a private group in Washington.