Iran would openly escalate foreign operations if attacked, further destabilizing Iraq and Afghanistan
While Iran is already suspected of supporting insurgents in Iraq and Afghanistan against the U.S. they have so far avoided direct confrontation. If the U.S. attacks Iran, they will be free to openly escalate their operations, leading to an all-out destabilizing war.
Quicktabs: Arguments
So far, Iran and its allies in the region have encouraged the Iraqi Shi'a to continue to show restraint and work for social stability in post-war Iraq, even in the face of what would otherwise be deemed egregious provocations, such as the bombing of Shi'a holy sites in Karbala, Najaf, and Baghdad that killed hundreds of Shi'a, including dozens of Iranian pilgrims. Many in Iran have reasoned that they will benefit greatly from a stable Iraq ruled by a Shi'a government, which will necessarily be closer to Iran by virtue of shared religious and historical affiliations. However, in the event of an attack on Iran, this calculus would likely yield to a desire for revenge. In such a scenario, Iranian Revolutionary Guards could cross the border in great numbers to promote a full-blown guerrilla war against the large U.S. presence in Iraq. Iranian intelligence agents, who are currently in Iraq in significant numbers, could provoke clashes between the U.S. forces and Shi'a majority, precipitating a general uprising against Coalition forces in Iraq. It is important to note that, unlike the foreign Salafi Jihadi fighters (a la Abu Musab al Zarqawi and his Tawhid network) who infiltrated Iraq to fight the Americans and are despised by the Iraqi Shi'a, Iranian infiltrators in Iraq are likely to be seen by Iraqi Shi'a in a very different light.
Any military action against Iran would also undermine America's nation-building efforts in Iraq and Afghanistan, due to possible Iranian retaliation in both countries. While Iranian efforts toward stabilizing these two states have been sporadic at best, and purposively obstructive at worst, there is little reason to doubt that Iran could make achieving US objectives in Iraq and Afghanistan far more difficult. Although mostly bluster, there is some truth to former Iranian President Ali Rafsanjani's argument that as long as American troops maintain a formidable presence on Iran's borders, "it is the United States that is besieged by Iran."3 The same holds true regarding Iran's ties to Hezbollah and its presence in Lebanon. By targeting Iran's nuclear program the United States would unwisely encourage Iranian escalation in a number of these arenas.
According to US intelligence officials, Iran provides Shia militias in Iraq with shoulder-fired missiles, multiple rocket launchers and rocket-propelled grenades. Iran also supports the radical Shia cleric Moqtada al-Sadr and his thousands of Mahdi Army loyalists, provides training and financial support to the Badr Brigade, and supports the country’s two largest Shia political parties, the Supreme Islamic Iraqi Council and Prime Minister Nuri al-Maliki’s Dawa Party.While Bush remains committed to Iraq, American military might may not be enough to compete with Tehran’s ‘hearts and minds’ campaign. Iran provides hospital treatment and surgery for wounded Iraqis, supplies Iraq with 2 million litres of kerosene a day, and provides 20% of Iraq’s cooking gas. Kenneth Katzman, a Middle East specialist for the Congressional Research Service, calls Iran’s wide-ranging leverage within Iraq ’strategic depth’, making the Iraqi government and populace acquiescent to Iranian interests.
One problem with a preventive war strategy is that Iran has the ability to retaliate in a number of ways. First among them is the prospect that Iran's political and military penetration of Iraq could lead to a rapid escalation of violence in that country and might well plunge the entire Persian Gulf region into chaos. In early 2006, U.S. intelligence warned of the most likely tactics Iran could employ: longrange missiles, secret commando units (presumably IRGC), and 'terrorist allies planted around the globe.' In particular, both the political and the security situations in Iraq could become nightmarish if the United States were to attack Iran. In January, powerful Shi'ite cleric Moqtada al-Sadr announced that if Iran were attacked, Sadr would throw his support behind Iran. Sadr's large militia, the Mahdi army, has clashed repeatedly with U.S. troops, and Sadr has become a major player in Iraq's national politics; he demonstrated his political influence most recently by prompting tens of thousands of supporters to take to the streets of Baghdad in August to profess support for Hizbullah leader Hassan Nasrallah's campaign against Israel by chanting 'Death to America! Death to Israel!' According to former National Security Council official Kenneth Pollack, Iran's Ayatollah Ali Khamenei has 'allowed the [Iranian] intelligence services to deploy to Iraq in force and position themselves to fight a war there if necessary.' Pollack concluded that if Iran decided to ratchet up its activity inside Iraq, our troubles in that country would 'increase dramatically, perhaps even insurmountably.' U.S. officials confirm this account, noting that Iranian agents have poured money and personnel into southern Iraq in an effort to create a 'greater Iran.' England's Chatham House think tank went so far as to argue that 'Iran has superseded [the United States] as the most influential power in Iraq.'
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.