The US could unleash vastly superior firepower if it attacked Iran but Tehran could strike back against its forces in Iraq and threaten oil supplies crucial to the world economy.
Speculation is growing that President George W. Bush could launch military action before he leaves office in January 2009 even though Washington says it is committed to resolving the crisis over Iran's disputed atomic ambitions diplomatically.
"It should be a walkover militarily," said London-based defence analyst Andrew Brookes about any US attempt to knock out the Islamic Republic's atomic installations.
"The hard bit is what comes afterwards and that is opening Pandora's box," said Mr Brookes of the International Institute for Strategic Studies (IISS) think-tank.
Western powers suspect Iran is seeking to build atom bombs. Iran says its nuclear programme is aimed at generating electricity so that it can export more of its oil and gas.
A former Iranian official with links to the country's highest authority, Supreme Leader Ayatollah Ali Khamenei, suggested Tehran would respond by using allies in the region to take the fight elsewhere in the Middle East.
"If they want to play games with us, I believe in a few ways we can turn Iraq into a fiery battlefield," he said.
Security experts voiced different opinions about the strength of Iran's armed forces in a showdown with the US, which they believe would involve a US air campaign but not an invasion by ground forces.
The military, under an arms embargo imposed by Washington, still partly relies on fighter aircraft and hardware bought before the 1979 Islamic revolution that toppled the US-backed shah, topped up with domestically produced equipment as well as imports from Russia and others.
A Western diplomat said Iranian leaders were confident US aerial bombardment would not threaten their hold on power.
"A bombing campaign has never removed a government and especially not in a country like this when there is no organised opposition," the Tehran-based diplomat said.
Iran's confidence has grown as it watched America's failure to get a grip on Iraq despite its overwhelming military supremacy.