Depending on the type and scale of the Iranian reaction to a possible Israeli strike, and whether it includes attacks on U.S. forces or bases, President Barack Obama would be under enormous domestic political pressure to come to Israel’s aid.

WASHINGTON — An Israeli pre-emptive attack on Iran’s nuclear sites could draw the U.S. into a new Middle East conflict, a prospect dreaded by a war-weary Pentagon seeking to avoid 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 U.S. 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 U.S. 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 superior 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 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 terrorists 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, possibly with strikes against the Iranian navy or land targets.

Michael O’Hanlon, a defense analyst at the Brookings Institution, sees a chance that the U.S. could largely stay out of the fight if Israel strikes first. If Iran’s air defenses manage to knock down an Israeli fighter pilot, however, U.S. special operations forces might be sent to rescue 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,” 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 defense minister, in a warning broadcast Saturday on state-run television, said a strike by “the Zionist regime will undoubtedly lead to the collapse of this regime.” However, Gen. 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 U.S. got drawn into the conflict, is at the core of U.S. officials’ rationale for publicly casting doubt on the wisdom of Israeli military action now.

Gen. 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 U.S. 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 defense 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.

“That’s kind of the nightmare scenario,” says Charles Wald, a retired Air Force general who argues nonetheless that the best hope for stopping Iran from getting the bomb is to strengthen the credibility of threats to use U.S. or Israeli military force. Such threats, he argues, could change Iran’s course.

The U.S. has two aircraft carriers, the USS Abraham Lincoln and the USS Carl Vinson, and other warships near Iran’s shores, as well as a wide array of warplanes at land bases on the Arabian Peninsula, and thousands of troops in Kuwait. It also has special operations forces near Iran’s eastern border, in Afghanistan.

Wald is co-leader of the Bipartisan Policy Center, which warned in a Feb. 8 report that Iran is “fast approaching the nuclear threshold.” While not advocating an Israeli pre-emptive strike, Wald’s group said the U.S. should provide Israel with 200 advanced GBU-31 bombs capable of reaching targets buried deep underground and three KC-135 refueling planes to extend the range of Israel’s strike jets.

The U.S. has no immediate plans to provide Israel with new military aid.

The consensus view among U.S. intelligence agencies is that Iran is not building a nuclear bomb now but is developing the capability to do so in the future. A critical question is how long it would take Iran to assemble a bomb, once a decision was made to proceed, and how much additional time it would need to affix the bomb to a missile or other means of delivering it beyond its own borders.

Obama has not ruled out using force to stop Iran from building a bomb. But his administration, joined by many allied nations, has counseled Israel to hold off. Several senior administration officials have been to Israel in recent days to emphasize caution, including Obama’s national security adviser, Tom Donilon.

Obama is due to meet with Prime Minister Benjamin Netanyahu at the White House on March 5. Defense Minister Ehud Barak is meeting Wednesday at the Pentagon with Defense Secretary Leon Panetta.

Iran insists that its nuclear program is for peaceful purposes and has invited the U.S. 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 Feb. 2 Washington Post report that Defense Secretary Leon 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 does not think Israel has yet decided to act.

In the U.S. view, any Israeli attack could set back the Iranian nuclear program a few of years at most, while giving Iranian leaders extra incentive and domestic support for rebuilding a clandestine program out of reach of U.N. inspectors.