Iran is not as helpless as you think. They may not be able to completely stop a US invastion, but they certainly could make the US pay for attacking them.
QUOTE(iranbodycount.org)
US action to pre-empt obvious Iranian responses, such as affecting tanker traffic through the Straits of Hormuz or moving Revolutionary Guard elements into parts of Iraq, could well mean that there would be immediate if apparent indications of comprehensive US military success in doing serious damage both to Iran’s presumed nuclear weapons development potential and in countering immediate Iranian responses. This could turn out to be as misleading as the early apparent successes in Iraq following regime termination within three weeks of the start of that war in March 2003. In fact, Iran has many options available in response, even if they are not options of immediate effect.
Straits of Hormuz: While one major aim of any US military action would be to forestall Iranian interference with Gulf oil exports, this would have to be near total in its effect on Iranian capabilities. This would be difficult if not impossible to achieve, leading to a fear of attack which alone would have a formidable impact on oil markets.
West Gulf oil facilities: Furthermore, it would be possible for paramilitary units linked to Iran to develop the ability to sabotage oil export facilities in western Gulf states such as Kuwait, Saudi Arabia and the United Arab Emirates. High levels of security would undoubtedly be maintained in these states, yet determined paramilitary groups would be difficult to control with certainty. Even one or two incidents of sabotage would raise tensions and further affect oil markets.
Revolutionary Guard: The Revolutionary Guard remains a strong if largely free-standing component of the Iranian defence system. While its facilities on the Persian Gulf coast and close to the border with Iran might be damaged in the early waves of US attacks, there would also be a very substantial base of support for the Guard, expressed by immediate improvements in morale, a greatly enhanced ability to recruit, and a determination to respond. Although US military action against Guard facilities might be undertaken to “warn off” the Guard from interfering in Iraq, the effect would almost certainly be shortlived, and the numerous links which already exist between Guard units and Iraqi Shi’a militias would be activated rapidly. Such demonstrable Iranian involvement in the Iraqi insurgency would result in an escalating US military response involving cross-border attacks on Iranian logistics. This would increase Iranian civilian casualties, cause economic disruption and also further increase internal Iranian support for the current regime.
Overall, and given the nature of the Iran/Iraq border, Iran would be in a very strong position to aid elements of the Iraqi insurgency in numerous ways, providing a wide range of armaments as well as personnel. This would give a substantial boost to an insurgency that, even three years after the termination of the old regime, is as active as ever.
International support: Given recent major long-term economic agreements between Iran and China, and also between Iran and India, as well as close links with Russia, a US attack would attract major criticisms, including from two of the five permanent members of the UN Security Council – China and Russia. The current Russian administration might prefer privately to see US military action avoided, but it would be in a very difficult position in relation to many of its neighbouring allies if it were not to condemn US military action against Iran most strongly, especially if this escalated to a protracted conflict.
The site is about the US attacking Iran to stop it from obtaining nuclear weapons, but I think the Iranian responses section is relevant here.
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As of January 2005, there are some 250,000 soldiers, sailors, airmen, Marines, and Coast Guardsmen deployed in support of combat, peacekeeping, and deterrence operations. This figure does not include those forces normally present in Germany, Italy, the United Kingdom or Japan unless bases at those locations are actively supporting a combat operation. Furthermore, tours of duty in these locations are routine and not considered hardship tours. If one were to include these forces the number of deployed troops worldwide would be around 350,000.
Not to mention, troops need to be kept at home...