Pressure is building on Iran. This week Europe agreed to new sanctions and President Bush again suggested something more serious – possible military strikes – if the Islamic Republic doesn't bend to the will of the international community on its nuclear program.

But increasingly military analysts are warning of severe consequences if the US begins a shooting war with Iran. While Iranian forces are no match for American technology on a conventional battlefield, Iran has shown that it can bite back in unconventional ways.

Iranian networks in Iraq and Afghanistan could imperil US interests there; American forces throughout the Gulf region could be targeted by asymmetric methods and lethal rocket barrages; and Iranian partners across the region – such as Hezbollah in Lebanon – could be mobilized to engage in an anti-US fight.

Iran's response could also be global, analysts say, but the scale would depend on the scale of the US attack. "One very important issue from a US intelligence perspective, [the Iranian reaction] is probably more unpredictable than the Al Qaeda threat," says Magnus Ranstorp at the Center for Asymmetric Threat Studies at the Swedish National Defense College in Stockholm.

"I doubt very much our ability to manage some of the consequences," says Mr. Ranstorp, noting that Iranian revenge attacks in the past have been marked by "plausible deniability" and have had global reach.

"If you attack Iran you are unleashing a firestorm of reaction internally that will only strengthen revolutionary forces, and externally in the region," says Ranstorp. "It's a nightmare scenario for any contingency planner, and I think you really enter the twilight zone if you strike Iran."

Though the US military has since early 2007 accused Iran's Qods Force – an elite element of the Islamic Revolutionary Guard Corps (IRGC) – of providing anti-US militias in Iraq with lethal roadside bombs, and of training and backing "special groups" in actions that the US government alleges have cost "thousands" of lives, US commanders have played down Iran's military capabilities.

Even Admiral William Fallon, who publicly opposed a US strike on Iran before he resigned in April, dismissed Iran as a military threat. "Get serious," Adm. Fallon told Esquire in March. "These guys are ants. When the time comes, you crush them."

But that has not kept Iran from rhetorical chest-beating, with an active military manpower of 540,000 – the largest in the Middle East – dependent on some of the lowest per capita defense spending in the region. Iran "can deal fatal blows to aggressor America by unpredictable and creative tactical moves," the senior commander Brig. Gen. Gholam Ali Rashid said in late May. "It is meaningless to back down before an enemy who has targeted the roots of our existence."

Iran's supreme religious leader Ayatollah Sayyed Ali Khamenei also warned of far-reaching revenge in 2006. "The Americans should know that if they assault Iran, their interests will be harmed anywhere in the world that is possible," he said. "The Iranian nation will respond to any blow with double the intensity."

Analysts say Iran has a number of tools to make good on those threats and take pride in taking on a more powerful enemy. "This is not something they are shying away from," says Alex Vatanka, a Middle East security analyst at Jane's Information Group in Washington.

"They say: 'Conventional warfare is not something we can win against the US, but we have other assets in the toolbox,' " says Mr. Vatanka, noting that the IRGC commander appointed last fall has been "marketed as this genius behind asymmetric warfare doctrine."

"What they are really worried about is the idea of massive aerial attacks on literally thousands of targets inside Iran," says Vatanka, also an adjunct scholar at the Middle East Institute. "Their reading of America's intentions in that scenario would be twofold: One is to obviously dismantle as much as possible the nuclear program; and [the other], indirectly try to weaken the [Islamic] regime."

Any US-Iran conflict would push up oil prices, and though Iran could disrupt shipping lanes in the Persian Gulf, its weak economy depends on oil revenues.

But nearby US forces in Iraq, Afghanistan, and the Gulf provide a host of targets. Iran claimed last October that it could rain down 11,000 rockets upon "the enemy" within one minute of an attack and that rate "would continue."

Further afield, Israel is within range of Iran's Shahab-3 ballistic missiles, and Hezbollah claims its rockets – enhanced and resupplied by Iran since the 2006 war to an estimated 30,000 – can now hit anywhere in the Jewish state, including its nuclear plant at Dimona.

Closer to home, Iran has honed a swarming tactic, in which small and lightly armed speedboats come at far larger warships from different directions. A classified Pentagon war game in 2002 simulated just such an attack and in it the Navy lost 16 major warships, according to a report in The New York Times last January.

"The sheer numbers involved overloaded their ability, both mentally and electronically, to handle the attack," Lt. Gen. K. Van Riper, a retired Marine Corps officer who commanded the swarming force, told the Times. "The whole thing was over in five, maybe 10 minutes."

During the 1990s, Iranian agents were believed to be behind the assassinations of scores of regime opponents in Europe, and German prosecutors issued an arrest warrant for Iran's intelligence minister.

Iran and Hezbollah are alleged to have collaborated in the May 1992 bombing of the Israeli Embassy in Buenos Aires in revenge for Israel's killing of a Hezbollah leader months before. Argentine prosecutors charge that they jointly struck again in 1994, bombing a Jewish community center in the Argentine capital that killed 85, one month after Israel attacked a Hezbollah base in Lebanon.

With some 30,000 on the payroll by one count, Iranian intelligence "is a superpower in intelligence terms in the region; they have global reach because of their reconnaissance ability and quite sophisticated ways of inflicting pain," says Ranstorp. "They have been expanding their influence.… Who would have predicted that Argentina would be the area that Hezbollah and the Iranians collectively would respond?"

Past examples show that "Tehran recognizes that at times its interest are best served by restraint," says a report on consequences of a strike on Iran published this week by Patrick Clawson and Michael Eisenstadt of The Washington Institute for Near East Policy.

But Iran could target the US, too, depending on the magnitude of any US strike. "Iran's capacity for terror and subversion remains one of its most potent levers in the event of a confrontation with the United States," says the report, adding that "success" in delaying Iran's nuclear programs could backfire.

If "US and world opinion were so angered by the strikes that they refused to support further pressure against Iran's nuclear ambitions, then prevention could paradoxically [eventually ensure] Iran's open pursuit of nuclear weapons," concludes the report.

And the long list of unconventional tactics should not be taken for granted in Tehran, says Vatanka, noting that the Islamic system's top priority is survival.

"So the Iranians have to be careful," says Vatanka. "Just because the US doesn't have the will right now, or the ability to produce the kind of stick that they would fear, doesn't mean the way of confrontation is going to pay off for them in the long run."