The U.S. military is developing plans for open warfare against Iran. Planning of this type is routine. Planning for war is always underway, with the most likely enemies being the most researched. During the Cold War, plans were constantly being changed for potential war with the Soviet Union. Today, it would be irresponsible for the U.S. military NOT to have plans for war against China or Iran. Just in case.
Iran is a potential enemy for a number of reasons. If open hostilities were to commence with Iran, it would make today’s overall war effort totally insufficient. The commitments of manpower in Iraq and Afghanistan could be lowered. However, this would not supply enough troops to make the outcome against Iran a certainty. A number of reasons are responsible.
Iraq is predominately desert. The terrain of Iran is vastly different from Iraq: A rugged, mountainous rim; A high, central basin with desert and mountains; small, discontinuous plains along both coasts. Mountain terrain is the worst environment in the world for a conventional force to engage in battle. The population of Iran is more than 3 times that of Iraq. These factors would demand a great deal more troops to be used in subduing Iran than Iraq. Today, the U.S. just does not have the armed troops that would be required to do the job well. We DO have the manpower. The problem is that they are in civilian clothes.
Without massive outside assistance, the U.S. would have to begin the draft in order to obtain the troop levels required to subdue Iran. Even then, this would require time. In order to expand the army this much, we would have to withdraw many experienced officers from active units to serve as cadres for the new units being formed. A large expansion would hurt many existing units’ abilities. The experienced men would help teach the new personnel their jobs in the newly forming units. These new units would take months to form and even longer to reach the level where they are ready to be committed to combat. It can take up to a full year to form a new combat ready division from scratch.
In summary, these factors make Iran a difficult proposition for the U.S. military. It can be done, but toppling the government of Iran would require a great deal more time than Iraq. It would also be far more expensive in terms of losses among U.S. units. I find war against Iran as being not very likely, UNLESS Iran successfully deployed one or more WMD. The best phrase I can think of is: "No battle plan ever survives first contact with the enemy."