By definition all military campaigns plan for the worst case. Because any U.S. attack against Iran would likely provoke an Iranian military response, the U.S. plan must include taking out Iranian air defense and naval assets and Tehran's robust ballistic missile capabilities.
Iran doesn't stand much chance of directly defending against a U.S. air attack. Much of its defense is outdated -- its air force comprises a few hundred fighters, many of them poorly maintained, including Soviet-era MiG-29s and U.S.-made F-14s that predate the collapse of U.S.-Iran relations three decades ago.
Iranian naval strength is divided into two main parts. One is the regular navy and the other is the naval branch of the Iranian Revolutionary Guard. Both forces have been updating and improving their equipment over the years.
Iran has been going through a naval buildup in the last decade and has a fleet of Iranian- and Russian-manufactured submarines, a hovercraft fleet that was once the largest in the world, naval airborne units that include several helicopter squadrons, minesweepers and a large arsenal of anti-ship missiles. The submarine fleet also includes mini-submarines manufactured domestically in Iran.
Iran has also tested a series of "submarine-to-surface" anti-ship missiles. These seem to have raised some concern that Iran could disrupt the flow of oil through the Persian Gulf in the event of an attack.
What are Iran's other options? Tehran could activate Hezbollah cells, including networks of financial supporters in the United States; encourage street protests in the Islamic world; get the Shiite militias in Iraq they support to attack U.S. and British troops in southern Iraq; possibly damage or destroy oil pipelines and other petroleum infrastructure, and even disrupt Persian Gulf shipping.
In the event of an attack the United States will have more responsibilities than just attacking Iranian nuclear facilities. It will have to keep the Strait of Hormuz open, international oil traffic running, and defend against Iranian missile attacks from land, air and sea.
The U.S. military should be worried. During a 2002 war game called Millennium Challenge, the opposing force, using a low-tech strategy of suicide planes and boats, was able to sink 16 U.S. Navy ships, killing thousands of U.S. sailors.
Iran is thought to have missile batteries at underground storage bunkers on the southwest of Abu Musa with HY-2 anti-ship missiles -- an advanced version of the Chinese Silkworm -- and Scud-C's. If the HY-2 is deployed on Abu Musa, it could hit any of the United Arab Emirates' ports, and U.S. naval forces do use Dubai's Jebel Ali port.
The Scuds are long-range surface-to-surface missiles and do not need to be moved off the mainland and to the islands in order to hit targets on the Arab side of the Gulf. In fact, the range of the Scud-C would enable it to hit the main population centers of the Gulf Arab states and Iraq.
Iran may also have other missiles such as the newer Chinese-made C-801 and C-802 anti-ship missiles. At the very least Iran could bring all of these missiles from the mainland to Abu Musa, the Tunbs, Sirri and other islands relatively quickly and store them there. Of course, to truly neutralize the missile threat from these islands U.S. forces would have to seize them, so at least in that sense ground forces would be required.
The Iranian arsenal also includes anti-ship missiles like the Kowsar. These are basically land-to-sea anti-ship missiles that can dodge electronic jamming systems.
These missiles could be used to disrupt shipping in the Strait of Hormuz. Indeed, Iran could fire missiles at oil and gas tankers, commercial cargo vessels, and U.S. and Gulf Cooperation Council naval vessels. Iran could also strike these targets by using fast patrol and fast attack naval craft that can also fire the C-801 and C-802. These craft, carrying these missiles, have been brought to the islands during Iranian patrols and naval exercises.
Iran could also rely on covert operations. Iran has midget submarines and combat divers and other assets like fifth-column forces that could be used in these covert operations. The islands could be used to launch these operations, too. During its war with Iraq in the 1980s Iran used the islands to launch fast interdiction boats and to launch helicopters in attacks against Saudi and Kuwaiti shipping and UAE offshore oil and gas fields from 1986 to 1988.
Iran could try to launch as much retaliation as possible from the mainland and islands near the Strait of Hormuz before its assets are crippled. This would seem especially likely if America's allies in the region facilitate the U.S. action. Given that U.S. forces are based in Qatar, Bahrain and Kuwait, and using docking facilities in Dubai and Muscat, there is a good chance Iran might respond with attacks against these targets.
Considering the many hundreds of targets the United States would have to hit in a short time, Iran could do a great deal of damage in the Gulf before its assets on the mainland and islands were destroyed. And any ocean mines they were able to lay before being hit could continue to do damage long after U.S. planes returned home.
The human costs in terms of civilian loss of life would be significant. At a minimum, thousands of Iranian civilians would be killed. Meanwhile, before its military capabilities were neutralized Iran would likely be able to wreak considerable damage on neighboring oil-producing Gulf states, severely disrupt the transportation of oil through the Persian Gulf, and support increased attacks against U.S. troops in Iraq, not to mention U.S. targets around the world.