The assessment said extended U.S. strikes could destroy Iran's most important nuclear facilities and damage its military forces but would only delay not stop the Islamic republic's pursuit of a nuclear bomb.
"You can't kill intellectual power," said retired Army Lt. Gen. Frank Kearney, who endorsed the report. He is a former deputy director at the National Counterterrorism Center and former deputy commander of U.S. Special Operations Command.
The report compiled by former government officials, national security experts and retired military officers is to be publicly released Thursday. It says achieving more than a temporary setback in Iran's nuclear program would require a military operation including a land occupation more taxing than the Iraq and Afghanistan wars combined.
An advance copy of the report was provided to The Associated Press.
The assessment emerges against the backdrop of escalating tensions between Israel and the U.S. over when a military strike on Iran might be required. The Israelis worry that Iran is moving more quickly toward a nuclear capability than the United States believes. The U.S. has not ruled out attacking but has sought to persuade Israel to give diplomacy more time.
Israel views a nuclear-armed Iran as a mortal threat, citing Iran's persistent calls for the destruction of the Jewish state, its development of missiles capable of striking Israel and Iranian support for Arab militant groups.
Tehran insists its nuclear program is for peaceful purposes only.
An oft-stated argument against striking Iran is that it would add to a perception of the U.S. as anti-Muslim a perception linked to the U.S.-led invasions of Iraq and Afghanistan and hardened by Internet-based video excerpts of an anti-Muslim film that may have fueled Tuesday's deadly attack on a U.S. diplomatic office in Libya.
"Planners and pundits ought to consider that the riots and unrest following a Web entry about an obscure film are probably a fraction of what could happen following a strike by the Israelis or U.S. on Iran," retired Lt. Gen. Gregory Newbold, an endorser of the Iran report and a former operations chief for the Joint Chiefs of Staff, said in an interview.
The report was compiled and endorsed by more than 30 former diplomats, retired admirals and generals and others who said their main purpose was to provide clarity about the potential use of military force against Iran. They reached no overall conclusion and offered no recommendations.
"The report is intended to have what we call an informing influence and hopefully something of a calming influence, but that's something readers will have to answer for themselves," said Thomas Pickering, a former U.S. ambassador to the United Nations who has held informal contacts with Iranian officials as recently as the past few months.
Kearney said the assessment was meant to stimulate thinking in the U.S. about the objectives of a military attack on Iran beyond the obvious goal of hitting key components of Iran's nuclear program. "Clearly there is some (U.S.) ability to do destruction, which will cause some delay, but what occurs after that?" he said in an interview.
Other endorsers of the report include Brent Scowcroft, who was President George H.W. Bush's national security adviser; former deputy secretary of state Richard Armitage, former Sens. Sam Nunn and Chuck Hagel and two retired chiefs of U.S. Central Command, Marine Gen. Anthony Zinni and navy Adm. William J. Fallon.
The analysis includes stark assertions about one of the most volatile and complex issues facing the U.S. in a presidential election year. President Barack Obama's failure to get Iran to negotiate acceptable limits on its nuclear program is cited by his opponents as emblematic of a misguided and weak foreign policy.
The report said the Obama administration's stated objective shared by Republican presidential nominee Mitt Romney of preventing Iran from obtaining a nuclear bomb is unlikely to be achieved through military force if action is limited to a combination of airstrikes, cyberattacks, covert operations and special operations strikes.
It says an extensive U.S. military assault could delay for up to four years Iran's ability to build a nuclear weapon. It also could disrupt Iranian government control, deplete its treasury and raise internal tensions.
"We do not believe it would lead to regime change, regime collapse or capitulation," it said, adding that such an attack would increase Iran's motivation to build a bomb, in part because the Iranian leadership would see building a bomb as a way to inhibit future U.S. attacks "and redress the humiliation of being attacked."
A more ambitious military campaign designed to oust the Iranian regime of hardline clerics or force an undermining of Iran's influence in the Mideast would require the U.S. to occupy part or all of the country, the report said.
"Given Iran's large size and population, and the strength of Iranian nationalism, we estimate that the occupation of Iran would require a commitment of resources and personnel greater than what the U.S. has expended over the past 10 years in the Iraq and Afghanistan wars combined," the report said.
The U.S. had as many as 170,000 troops in Iraq at the height of the 2003-10 war, and U.S. troop levels in Afghanistan peaked last year at 100,000. Eleven years into the Afghan war the U.S. still has about 74,000 troops there.
Early drafts of the report were coordinated by the nonpartisan Iran Project, a private group funded in part by the Rockefeller Brothers Fund, a philanthropy that promotes peace and democracy. The final version includes contributions from others with national security expertise. It is based on publicly available documents, including unclassified intelligence reports.