The earlier post Iran described some of the threats the Mullahs may pose to the United States. In general most of the direct threats are not very serious. The threat to 'set the Middle East ablaze' should the US pre-emptively strike Iranian WMD development facilities is pretty pathetic. Supposed instructions to "Revolutionary Guards sectors to respond swiftly - within no more than an hour and without waiting for orders - against pre-selected targets" will almost certainly rely on prepositioned terrorist cells in the absence of any real delivery systems and while this may kill a few hundred people it will hardly put a dent in the fighting power of the American armed forces. The threat of an electromagnetic pulse attack on the US by an Iranian nuclear weapon delivered by missile at high altitude is unlikely to materialize in the short term; and if it did, would originate from an identifiable source. As the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack noted on page 2 those threats are most dangerous when their origin cannot be traced.
EMP effects from nuclear bursts are not new threats .... The Soviet Union in the past ... are potentially capable of creating these effects ... mixed with ... nuclear devices that were the primary source of destruction, and thus EMP as a weapons effect was not the primary focus. Throughout the Cold War, the United States did not try to protect its civilian infrastructure against either the physical or EMP impact of nuclear weapons, and instead depended on deterrence for its safety.
An Iranian EMP device detonated at high altitude over the US lacks the chief advantage of a terrorist nuclear weapon: deniability. Its point of origin would be computed before it completed its flight and would easily be considered a nuclear attack on US soil to be met with massive retaliation. Whether through Revolutionary Guards or missiles, the Mullahs on the whole don't have many good ways of directly attacking the United States and they know this. Their efforts have therefore been focused on acquiring nuclear weapons as a deterrent so that they can safely pursue a program of indirect, terrorist warfare on the US. Their intent is being dictated by their capability.
The more capable US Armed Forces could directly attack the regime in Teheran but its deployments suggest otherwise. A map of the population densities of Iran is shown below (hat tip: Microsoft Encarta), with the more densely populated areas in darker red. The population centers of Iran are in an arc embracing the Caspian Sea behind the rampart of the Zagros mountains to the south and the Elberuz mountains to the north. The 3+ US divisions in Iraq are arguably in the worst place from which to open a land campaign against Iran because they are on the wrong side of Zagros mountain barrier relative to the centers of Iranian power. It might be possible to campaign across the Zagros, around Lake Urumia in the north, for example, and descend on the Tabriz-Teheran road, but it doesn't look easy. During the Iraq-Iran War, Saddam Hussein's forces never made a serious attempt to cross the Zagros into the Iranian interior but concentrated instead on attempting to secure Iran's access to the the Persian Gulf. But unlike Saddam, the US already controls Iran's access to the Gulf by naval force and has no real need to seize its port cities.
It is reasonable to speculate that while the US will improve its capability to attack directly, it is really deployed to confront the Iranian regime indirectly. US organizing efforts in Kurdistan, Afghanistan and in Central Asia have opened clandestine highways into Iran. The game of infiltration and counter-infiltration is apparent in Iraq. An earlier post described the activities of the Iranian-sponsored Badr Corps in Iraq through which the Mullahs may hope to wage an intelligence/terrorist campaign against the US. But just as the enemy has tried to subvert Iraq by infiltrating its security forces the Badr Corps also provides a pathway back into the Mullahcracy for US agents. Agent networks are doors which swing both ways.
As the fall of the Soviet Union and the Syrian retreat from Lebanon illustrated, indirect warfare can go on for a long time until a 'key' issue or event presents itself which precipitates the actual fall of a regime. It would be fair to say that no one could predict the precise place where the totalitarian system will break -- Berlin in the case of the USSR or the Hariri assassination in the case of Syria -- but that it was important to maintain continuous pressure and to be opportunistically ready to turn the 'key' when it presented itself. Perhaps the principal difference between Carter and Reagan; Clinton and Bush was that the latter of each pair was waiting for the lock to turn while the former were uninterested.
When Richard Perle testified before House Armed Services Committe in April 2005 he summed up what he had learned from the Iraq campaign. None of his regrets had to do with military shortcomings. The deficiencies in the American campaign were in the political sphere. He spoke of the need to create indigenous groups sympathetic to democratic aims before taking on a tyranny and of involving them immediately in the governance of the country.
First, it is essential that we are clear about, and carefully align, our political and military objectives. ... American forces, working with the indigenous opposition to the Taliban regime, went into Afghanistan on October 7, 2001, less than a month after the attack of 9-11. ... We went in with a small force--never more than 10,000--and despite the criticism that the force was too small and that we were facing a quagmire as a result, some of which appeared in as little as three weeks, we quickly achieved our objective. ... In Iraq we succeeded in driving Saddam Hussein from office in three weeks. And while we were received in Iraq as liberators in the days following the collapse of Saddam's army and regime, we did not enjoy the benefit of a close collaboration with the indigenous opposition to his brutal, sadistic dictatorship.
This brings me to my second lesson: In aligning our political and military strategy, we should make sure we have the support of a significant segment of the local population. Even more, we should work with those whose interests parallel our own, taking them into our confidence and planning to operate in close collaboration with them.
The third lesson is, by now, generally accepted: our intelligence is sometimes, dangerously inadequate.
Although Perle was ostensibly discussing the Iraqi campaign, his reflections were not made in the context of a disinterested academic inquiry into past events but as lessons meant to be applied to future campaigns; i.e. Iran. This suggests that long before the US attempts a direct assault on the Iranian regime it will probably attempt to achieve each of the three things Perle mentioned: a relationship with a partner Iranian group; the development of a popular desire to overthrow the Mullahs; and a commanding intelligence picture.