WASHINGTON » A classified war simulation exercise held this month to assess the U.S. military’s capabilities to respond to an Israeli attack on Iran forecast that the strike would lead to a wider regional war, which could draw in the United States and leave hundreds of Americans dead, according to U.S. officials.

The officials said the so-called war game was not designed as a rehearsal for U.S. military action — and they emphasized that the exercise’s results were not the only possible outcome of a real-world conflict. But the game has raised fears among top U.S. planners that it may be impossible to preclude U.S. involvement in any escalating confrontation with Iran, the officials said.

In the debate among policymakers over the consequences of any possible Israeli attack, that reaction may give stronger voice to those within the White House, Pentagon and intelligence community who have warned that a strike could prove perilous for the United States.

The results of the war game were particularly troubling to Gen. James N. Mattis, who commands all U.S. forces in the Middle East, Persian Gulf and Southwest Asia, according to officials who either participated in the Central Command exercise or who where briefed on the results and spoke on condition of anonymity because of its classified nature. When the exercise had concluded earlier this month, according to the officials, Mattis told aides that an Israeli first strike would likely have dire consequences across the region and for U.S. forces there.

The two-week war game, called "Internal Look," played out a narrative in which the United States found it was pulled into the conflict after Iranian missiles struck a U.S. Navy warship in the Persian Gulf, killing about 200 Americans, according to officials with knowledge of the exercise. The United States then retaliated by launching its own strikes on Iranian nuclear facilities.

The initial Israeli attack was assessed to have set back the Iranian nuclear program by roughly a year, and the subsequent U.S. strikes did not slow the Iranian nuclear program by more than an additional two years. However, other Pentagon planners have said that America’s arsenal of long-range bombers, refueling aircraft and precision missiles could do far more damage to the Iranian nuclear program — if President Barack Obama were to decide on a full-scale retaliation.

The exercise was designed specifically to test internal military communications and coordination among battle staffs in the Pentagon; Tampa, Fla., home of the Central Command headquarters; and in the Persian Gulf in the aftermath of an Israeli strike. But the exercise was written to assess a pressing, potential, real-world situation.

In the end, the war game reinforced to military officials the unpredictable and uncontrollable nature of a strike by Israel, and a counterstrike by Iran, the officials said.

U.S. and Israeli intelligence services broadly agree on the progress Iran has made to enrich uranium. But they disagree on how much time there would be to prevent Iran from building a weapon if leaders in Tehran decided to go ahead with one.

With the Israelis saying publicly that the window to prevent Iran from building a nuclear bomb is closing, U.S. officials see an Israeli attack on Iran within the next year as a possibility. They have said privately that they believe that Israel would probably give the United States little or no warning should Israeli officials make the decision to strike Iranian nuclear sites.

Officials said that, under the chain of events, Iran believed that Israel and the U.S. were partners in any strike against Iranian nuclear sites and therefore considered U.S. military forces in the Persian Gulf as complicit in the attack. Iranian jets chased Israeli warplanes after the attack, and Iranians launched missiles at a U.S. warship in the Persian Gulf, viewed as an act of war that allowed an American retaliation.

Internal Look, long one of Central Command’s most significant planning exercises, is carried out about twice a year to assess how the headquarters, its staff and command posts in the region would respond to various real-world situations.

Over the years, it has been used to prepare for various wars in the Middle East. According to the defense website Globalsecurity.Org, military planners during the Cold War used Internal Look to prepare for a move by the Soviet Union to seize Iranian oil fields. The U.S. war plan at the time called for the Pentagon to march nearly six Army divisions north from the Persian Gulf to the Zagros Mountains of Iran to blunt a Soviet attack.

In December 2002, Gen. Tommy Franks, who was the top officer at Central Command, used Internal Look to test the readiness of his units for the coming invasion of Iraq.

Many experts have predicted that Iran would try to carefully manage the escalation after an Israeli first-strike in order to avoid giving the United States a rationale for attacking with its far superior forces. Thus, it might use proxies to set off car bombs in world capitals or funnel high explosives to insurgents in Afghanistan to attack U.S. and NATO troops. While using surrogates might, in the end, not be enough to hide Iran’s instigation of these attacks, the government in Tehran could at least publicly deny all responsibility.

Some military specialists in the United States and in Israel who have assessed the potential ramifications of an Israeli attack believe that the last thing Iran would want is a full-scale war on its territory. Thus, they argue that Iran would not directly strike U.S. military targets, whether warships in the Persian Gulf or bases in the region.

Their analysis, however, also includes the broad caveat that it is impossible to know the internal thinking of the senior Iranian leadership and is informed by the awareness that even the most detailed war games cannot predict how nations and their leaders will react in the heat of conflict.

Yet these specialists continue their work, saying that any insight on how the Iranians will react to an attack will help determine whether the Israelis launch a strike — and what the U.S. position will be if they do.

Israeli intelligence estimates, backed by academic studies, have cast doubt on the widespread assumption that a military strike on Iranian nuclear facilities would set off a catastrophic set of events like a regional conflagration, widespread acts of terrorism and sky-high oil prices.

"A war is no picnic," Defense Minister Ehud Barak told Israel Radio in November.

But if Israel feels itself forced into action, the retaliation would be bearable, he said.

"There will not be 100,000 dead or 10,000 dead or 1,000 dead," Barak said. "The state of Israel will not be destroyed."

© 2012 The New York Times Company