In late 2006, George W. Bush met with the Joint Chiefs of Staff at the Pentagon and asked if military action against Iran's nuclear program was feasible. The unanimous answer was no. Air strikes could take out some of Iran's nuclear facilities, but there was no way to eliminate all of them. Some of the nuclear labs were located in heavily populated areas; others were deep underground. And Iran's ability to strike back by unconventional means, especially through its Hizballah terrorist network, was formidable. The military option was never officially taken off the table. At least, that's what U.S. officials always said. But the emphasis was on the implausibility of a military strike. "Another war in the Middle East is the last thing we need," Secretary of Defense Robert Gates wrote in 2008. It would be "disastrous on a number of levels." (See pictures of President Bush in the Middle East.)

Gates is sounding more belligerent these days. "I don't think we're prepared to even talk about containing a nuclear Iran," he told Fox News on June 20. "We do not accept the idea of Iran having nuclear weapons." In fact, Gates was reflecting a new reality in the military and intelligence communities. Diplomacy and economic pressure remain the preferred means to force Iran to negotiate a nuclear deal, but there isn't much hope that's going to happen. "Will [sanctions] deter them from their ambitions with regards to nuclear capability?" CIA Director Leon Panetta told ABC News on June 27. "Probably not." So the military option is very much back on the table.

What has changed? "I started to rethink this last November," a recently retired U.S. official with extensive knowledge of the issue told me. "We offered the Iranians a really generous deal, which their negotiators accepted," he went on, referring to the offer to exchange Iran's 1.2 tons of low-enriched uranium (3.5% pure) for higher-enriched (20%) uranium for medical research and use. "When the leadership shot that down, I began to think, Well, we made the good-faith effort to engage. What do we do now?" (See pictures of terror in Tehran.)

Other intelligence sources say that the U.S. Army's Central Command, which is in charge of organizing military operations in the Middle East, has made some real progress in planning targeted air strikes — aided, in large part, by the vastly improved human-intelligence operations in the region. "There really wasn't a military option a year ago," an Israeli military source told me. "But they've gotten serious about the planning, and the option is real now." Israel has been brought into the planning process, I'm told, because U.S. officials are frightened by the possibility that the right-wing Netanyahu government might go rogue and try to whack the Iranians on its own. (Comment on this story.)

One other factor has brought the military option to a low boil: Iran's Sunni neighbors really want the U.S. to do it. When United Arab Emirates Ambassador Yousef al-Otaiba said on July 6 that he favored a military strike against Iran despite the economic and military consequences to his country, he was reflecting an increasingly adamant attitude in the region. Senior American officials who travel to the Gulf frequently say the Saudis, in particular, raise the issue with surprising ardor. Everyone from the Turks to the Egyptians to the Jordanians are threatening to go nuclear if Iran does. That is seen as a real problem in the most volatile region in the world: What happens, for example, if Saudi Arabia gets a bomb, and the deathless monarchy there is overthrown by Islamist radicals?

For the moment, the White House remains as skeptical as ever about a military strike. Most senior military leaders also believe Gates got it right the first time — even a targeted attack on Iran would be "disastrous on a number of levels." It would unify the Iranian people against the latest in a long series of foreign interventions. It would also unify much of the world — including countries like Russia and China that we've worked hard to cultivate — against a recowboyfied U.S. There would certainly be an Iranian reaction — in Iraq, in Afghanistan, by Lebanese Hizballah against Israel and by the Hizballah network against the U.S. and Saudi homelands. A catastrophic regional war is not impossible. (See who's who in Barack Obama's White House.)

Of course, it is also possible that this low-key saber-rattling is simply a message the U.S. is trying to send the Iranians: it's time to deal. There have been rumblings from Tehran about resuming negotiations, although the regime has very little credibility right now. The assumption — shared even by some of Iran's former friends, like the Russians — is that any Iranian offer to talk is really an offer to stall. A specific, plausible Iranian concession may be needed to get the process back on track. But it is also possible that the saber-rattling is not a bluff, that the U.S. really won't tolerate a nuclear Iran and is prepared to do something awful to stop it.