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But retired Navy Adm. Mike Mullen, former chairman of the Joint Chiefs of Staff, disclosed that an updated war plan exists.

“You’ve got to be careful of unintended consequences here,” Defense Secretary Leon E. Panetta said last week. “And those consequences could involve not only not really deterring Iran from what they want to do, but, more importantly, it could have a serious impact in the region, and it could have a serious impact on U.S. forces in the region.”

John Pike, who directs, said oil-producing Iran likely would retaliate against Israel directly through its surrogates — Hamas, the Islamist militant group that controls the Gaza Strip to Israel’s south, and the Shiite militia Hezbollah in Lebanon, on the Jewish state’s northern border.

“Attacking Persian Gulf shipping is tricky for them, since the Saudis are not completely dependent on the Persian Gulf, whereas Iran is,” Mr. Pike said. “But attacks would drive up the price of oil, which would benefit Iran.”

The hard-line Islamic state also could take the momentous step of attacking the U.S. homeland.

“If Iran was bold, they would infiltrate commandos across the Mexican border and blow up elementary schools in Iowa,” Mr. Pike said. “Attacking U.S. and Saudi targets is risky for them because there would still be lots of stuff in Iran that could be blown up in retaliation, and Iran would run out of things of value before the U.S. ran out of bombs.”

War drums began beating more loudly this month with reports of Mr. Netanyahu trying to win Cabinet approval for a strike on Iran. Mr. Ahmadinejad has called for Israel’s destruction.

But the Israeli military would face a daunting challenge.

Iran has so many scattered, buried nuclear sites — perhaps 50 or more —s that Israel’s air force and its U.S.-provided long-range F-15s would have a difficult time executing the kind of broad strategic campaign needed to hit most of them.

“Ultimately, it won’t be a successful mission because Iran has not concentrated its nuclear capabilities in one or two locations,” Mr. Johnson said. “They’ve dispersed them.

“And by virtue of dispersing them, Israel’s best chance is to get one or two targets. On top of this, [Israeli pilots] still have the problem of penetrating an integrated air defense system and doing so over a long distance.”

Mr. Johnson added: “If they were using nuclear weapons, they might have a chance of really causing significant damage to these sites.”

Mr. Pike has the view that if Israel is able to destroy just a handful of sites, it would set back Iran for a while.

“There are really not much more than half a dozen critical targets in Iran’s nuclear program,” he said. “Iran needs their entire complex, so Israel does not have to destroy everything in order to disable the program.”