WASHINGTON—When Iran was caught last September building a secret, underground nuclear enrichment plant at a military base near the city of Qum, the country's leaders insisted they had no other choice. With its nuclear facilities under constant threat of attack, they said, only a fool would leave them out in the open.

So imagine the surprise of international inspectors almost two weeks ago when they watched as Iran moved nearly its entire stockpile of low-enriched nuclear fuel to an above-ground plant. It was as if, one official noted, a bull's-eye had been painted on it.

Why take such a huge risk?

That mystery is the subject of fervent debate among many who are trying to decode Iran's intentions. The theories run from the bizarre to the mundane: Under one, Iran is actually taunting the Israelis to strike first. Under another, it is simply escalating the confrontation with the West to win further concessions in negotiations. The simplest explanation, and the one that the Obama administration subscribes to, is that Iran has run short of suitable storage containers for radioactive fuel, so it had to move everything.

The debate reflects the depth of confusion about the intentions of a badly divided Iranian leadership. Since October, when Iran agreed in principle to ship much of its nuclear stockpile out of the country so that it could be converted to fuel for a medical reactor, there have been a series of unexplained actions. President Mahmoud Ahmadinejad has veered from hailing the deal to backing away from it. The country has declared that it will soon build 10 new enrichment plants—a number it does not have the capacity to carry out. It has declared that it has answered all the questions posed by inspectors about potential work on weapons; the inspectors say there have been no responses since mid-2008.

So while Washington and its allies are deeply immersed in assessing Iran's technical capabilities, they are still trying to divine its political intentions. Despite considerable evidence that the United States and Israel have at least partly penetrated the Iranian program—snatching up scientists, obtaining photos of the inside of facilities and tapping into computer data from the nuclear program—they still are not certain whether Iran is seeking a nuclear bomb, or just the ability to build one, or even merely the appearance of the ability. As one senior adviser to President Barack Obama said late last year, “;We've got a near-perfect record of being wrong about these guys for 30 years.”;

What touched off this whole guessing game was a single sentence in one of the normally bone-dry reports of the International Atomic Energy Agency. The report said that on Feb. 14, with inspectors present, the Iranians moved roughly 4,300 pounds of low-enriched uranium out of deep underground storage to the small plant that they have declared they will use to re-enrich the fuel to 20 percent purity. (It takes 80- to 90-percent purity to make a weapon, a relatively small technological leap from 20 percent.)

On the surface, the move made no sense. Iran does not need anywhere near that much fuel for its ostensible purpose: feeding an aging reactor in Tehran that makes medical isotopes. Moreover, the fuel now sits out in the open, where an air attack, or even a carefully staged accident or fire, could destroy it.

American and European officials will say little on the record because the guessing game touches on three of the most sensitive subjects in the dispute: Whether Israel will strike the facilities and risk igniting a broader Middle East war; whether there is still time to stop the Iranian program through sanctions and diplomacy; and who is really in control of Iran and its nuclear program. “;There's no technical explanation, so there has to be some other motivation,”; one senior administration official who studies the Iranian strategy said after a White House briefing last week following the atomic agency's revelation.

The strangest of the speculations—but the one that is being talked about most—is that Iran's Islamic Revolutionary Guards Corps is inviting an attack to unify the country after eight months of street demonstrations that have pitted millions of Iranians against their government. As one senior European diplomat noted Thursday, an Israeli military strike might be the “;best thing”; for Iran's leadership because it would bring Iranians together against a national enemy.

It would offer an excuse some Iranians might sorely want to throw out the nuclear inspectors and renounce the Nuclear Nonproliferation Treaty. That would leave Iran in the position that North Korea is in: free to manufacture fuel or bombs without inspectors to blow the whistle.

Others, including some officials in the White House, say they do not buy that theory. Iran has worked too hard to let its supply be destroyed, they argue. “;I really doubt they are taunting the Israelis to hit them,”; said Kenneth Pollack, a scholar at the Brookings Institution who recently ran a daylong simulation of what would happen after an Israeli attack on Iran's nuclear facilities. “;It would be humiliating for the Iranian regime,”; he said. He speculated that Iran would have to retaliate, and “;the ensuing confrontation would go in directions no one can really predict.”;

Pollack numbers among those who suspect another explanation: brinkmanship. The Iranians have made clear that they do not like the terms their own negotiators came home with for swapping their nuclear fuel for specialized fuel for the medical reactor. By moving their fuel supply to the enrichment plant, they are essentially threatening to turn it all to near-bomb-grade fuel—and perhaps force the United States to reopen negotiations.

But the simplest explanation, that the Iranians had no choice, has its proponents. The fuel is stored in one big, specialized cask. When someone ordered that the fuel begin being fed into the giant centrifuges for further enrichment, engineers moved it to the only spot available—the exposed plant. Or, as one U.S. intelligence official said, “;You can't dismiss the possibility that this is a screw-up.”;

Whatever the cause, military officials say this is a tempting moment for the Israelis. The Obama administration clearly wants to make sure Israel does not take military action. In recent weeks it has sent the national security adviser and the chairman of the Joint Chiefs of Staff to Israel to ensure there are no surprises like Israel's 2007 strike on a nuclear reactor under construction in Syria. In that case, the Israelis gave the White House little warning of its decision to act.

Michael Slackman contributed reporting from Cairo and Amman, Jordan; Robert F. Worth from Beirut, Lebanon; and Mark Landler from Washington.