Washington: An Israeli pre-emptive attack on Iran's nuclear sites could draw the US into a new Middle East conflict, a prospect dreaded by a war-weary Pentagon wary of new entanglements.
That could mean pressing into service the top tier of American firepower — warplanes, warships, special operations forces and possibly airborne infantry — with unpredictable outcomes in one of the world's most volatile regions.
"Israel can commence a war with Iran, but it may well take US involvement to conclude it," says 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 could soon enter a "zone of immunity" in which enough of its nuclear materials are beyond the reach of Israeli air power so that it could not be stopped, or perhaps could be stopped only by superior American weapons.
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 in any number of ways. That possibly 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 in Gaza to hit Israel with missiles from closer range.
Iran may also block the Strait of Hormuz, a key transit route for the world's oil tankers. It could attack nearby Bahrain, home to the US Navy's 5th Fleet. In either of these scenarios, the US military would almost certainly hit back, possibly with strikes against the Iranian navy or land targets.
Michael O'Hanlon, a defence analyst at the Brookings Institution, sees a chance that the US could largely stay out of the fight if Israel struck first. If Iran's air defences managed to knock down an Israeli fighter pilot, however, US special operations forces might be sent to rescue him, he said.
If the US 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", O'Hanlon said. "I would bet against most other direct American involvement."
Iran's response to an Israeli pre-emptive strike is unpredictable. Iran's defence minister, in a warning broadcast on Saturday on state-run television, said a strike by "the Zionist regime will undoubtedly lead to the collapse of this regime". General Ahmad Vahidi did not say what type of action Iran would take should Israel attack.
Uncertainty about Iranian retaliation, as well as the cascade of potential consequences if the US got drawn into the conflict, is at the core of US officials' rationale for publicly casting doubt on the wisdom of Israeli military action now.
General Martin Dempsey, chairman of the Joint Chiefs of Staff, bluntly made the point last weekend. He told CNN's Fareed Zakaria GPS that the retaliation equation is "the reason that we think that it's not prudent at this point to decide to attack Iran" and "that's been our counsel to our allies, the Israelis, well-known, well-documented". He said he doubts Israel has been persuaded by Washington's pleadings.
Depending on the type and scale of the Iranian reaction to an Israeli strike, and whether it included attacks on US forces or bases, President Barack Obama would be under enormous domestic political pressure to come to Israel's aid. His prospective Republican challengers for the White House have tried to portray Obama as insufficiently loyal to Israel and overly tolerant of Iran.
Obama could decide to provide Israel with extra missile defence systems, such as the Patriot, to help defend its cities. He could choose a more aggressive course, ordering follow-up air strikes on Iranian targets such as military bases and its remaining nuclear facilities.
US President Barack Obama has not ruled out using force to stop Iran from building a bomb. But his administration has advised Israel to hold off. Several senior administration officials have been to Israel in recent days to emphasise caution, including Obama's national security adviser, Tom Donilon.
Obama is due to meet with Israeli Prime Minister Benjamin Netanyahu at the White House on March 5. Israeli defence minister Ehud Barak is meeting with Defence Secretary Leon Panetta at the Pentagon on Wednesday.
Iran insists that its nuclear programme is for peaceful purposes and has invited the US and four other powers to sit down for nuclear talks. But in recent weeks tensions have grown amid Iranian threats to close the Strait of Hormuz in retaliation for Western penalties and debate in Israel about a pre-emptive strike.
Adding to a sense of urgency was a February 2 Washington Post report that Panetta believes there is a strong likelihood that Israel will attack Iran in April, May or June. Panetta has not disputed the report but has said he doesn't think Israel has yet decided to act.