Iran's offer of peace and the US's Hubris.
According to Gareth Porter of Antiwar.com, back in 2003 Iran had offered peace to the United States and Israel in return for security assurances from the US. Unsurprisingly, the offer came in April or May of that year, just weeks after the US military had strolled to Baghdad.
Iran blinked when they offered peace but the US was too arrogant to realise it. They took it as a sign of weakness and set their sites on marching to Tehran from Baghdad. Saddam's army had folded in 3 weeks so the Iranians would be no great shakes. However now, after 3 years of being softened up by the sunni arab Guerilla insurgency in Iraq, they are in no fit state to step into the ring with Iran.
While the US is the stronger and heavier fighter who undoubtedly can still cause damage to its lighter opponent, it steps into the ring with weak knees. Under normal circumstances Iran may have folded under a US bombing campaign, but with 150,000 US hostages in iraq as Iraqi blogger Riverbend put it, they will be supremely confident of hurting the US.
Also, the mess that is iraq has detered any Anti-regime elements in Iran from working against the state. As bad as they see things, there is no way they want the chaos to be found next door. The people will rally around the flag instead of turning against their leaders. What the US hasnt grasped is the strong Iranian nationalism that seems to transcend ethnicity or political ideology. EVERY iranian ive spoken to from Khomeinists to shah loyalists, insist that Iran has the right to a nuclear programme, which of course they do under international law.
The reason iraqis allowed the invasion was because the Baathist regime had treated them so horrendously. The clerics in Iran, love them or hate them, simply havent engaged in that sort of oppression, nor do I believe is it their intention to. For an "axis of evil" state, its political landscape is quite diverse, as opposed to Saddams strict idol worship state. If america thinks that internal opposition to the Iranian clerics is similar to that faced by saddam, they are grossly mistaken.
They would do well to ignore the sun soaked exiles in LA and the Manucher Ghorbanifars' of this world, when they tell the Bush Administration that they will be greeted by cheering crowds. Otherwise, as Juan Cole rightly points out, Arlington cemetary will have a busy century.