An Israeli pre-emptive attack on Iran's nuclear sites could draw the U.S. into a new Mideast conflict, a prospect dreaded by a war-weary Pentagon.
That could mean using the top tier of U.S. firepower -- warplanes, warships, special operations forces and maybe airborne infantry -- with unpredictable outcomes in one of the most volatile regions.
"Israel can commence a war with Iran, but it may well take U.S. involvement to conclude it," said Karim Sadjadpour, a Middle East specialist at the Carnegie Endowment for International Peace.
An armed clash with Iran is far from certain. Diplomacy backed by increasingly tough economic penalties is still seen by the United States and much of the rest of the world as worth pursuing for now, not least because the other options -- going to war or simply doing nothing -- are considered more risky.
Israel, however, worries that Iran soon could enter a "zone of immunity" in which enough of its nuclear materials are beyond the reach of Israeli air power so that Iran could not be stopped, or perhaps could be stopped only by American firepower.
If Israel's American-made strike planes managed to penetrate Iranian air space and bomb Iran's main nuclear facilities, some of which are underground, then Iran would be expected to retaliate. That could include the firing of Shahab-3 ballistic missiles at Tel Aviv or other Israeli targets.
Iran might take a less direct approach, relying on its Hezbollah allies in Lebanon or Hamas militants in Gaza to hit Israel with missiles from closer range.
Iran also might block the Strait of Hormuz, a key transit route for the world's oil tankers. It could attack nearby Bahrain, home to the U.S. Navy's 5th Fleet. In either of these scenarios, the U.S. military almost certainly would hit back.
Michael O'Hanlon, a defense analyst at the Brookings Institution, sees a chance that the U.S. could stay out of the fight if Israel struck first. But if Iran's air defenses knocked down an Israeli fighter pilot, U.S. special operations forces might be sent to get him, he said.
If the U.S. spotted Iran preparing to fire a ballistic missile at Israel in a retaliatory act, "it's possible we would decide to take that missile out," he said. "I would bet against most other direct American involvement."
Iran's response to an Israeli pre-emptive strike is unpredictable. Iran's defense minister, in a warning broadcast Saturday, said a strike by "the Zionist regime will undoubtedly lead to the collapse of this regime." Gen. Ahmad Vahidi did not say what type of action Iran would take if Israel attacked.