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I really do not believe there will be a direct war between Iran and the United States at least through the end of Obama's term in office. If there is a war, I expect it to be historically disastrous for the United States, and that it will be written in future history books as the war that ended the US empire.
How could a war start? Let's say the US interprets either the sanctions resolutions in place or some new sanctions resolution as permitting US interception of Iranian ingoing shipping, and by holding up supplies arriving to Iran begins seriously hampering the Iranian economy. I use this scenario to point out that the US does not need more Security Council resolutions to impose crippling sanctions if it wants to. The US, right now, just does not want to because they would start a war the US does not want. Alternatively we can start the war by Israel or the US bombing any kind of targets in Iran.
But with the shipping scenario, Iran responds to what Iran, rightly, Security Council resolution or not, interprets as an act of war by escalating its support for anti-US forces in Pakistan, Afghanistan and Iraq. Because there has not been an overt bombing yet, Iran does not directly use Iranian forces, but provides material and expertise in attacking US supply lines in these three countries.
Now Iran is under crippling sanctions, and eventually US soldiers in Iraq and Afghanistan are also under crippling sanctions. Either the US will allow anti-US forces to hold sanctuary in Iran as they continue their escalating and increasingly effective campaign of preventing supplies from reaching US bases or the US will attack supply depots and training areas in Iran. Iran will become more brazen, though still indirect, in its attacks on US bases and their supplies until the US actually attacks Iran. Once this happens, it is the same as if the US had bombed Iran's nuclear installations as a first move, except that in this scenario, Iran has had more time to establish its connections with anti-US forces in the three countries.
Once the actual overt attack on Iran happens, Iran is able to move its conflict with the United States into the open. Now there are Iranian units operating at least in Afghanistan and Iraq, possibly also in Pakistan, but possibly Iran will continue operating through proxies in Pakistan. Also, because the US escalated hostilities and attacked Iran against the advice and request of the Iraqis and Afghans, Maliki and Karzai, the political will to help the US fend off the Iranians, even if it was possible to do so, does not exist in either country, especially Iraq. The US cannot count on Iraqi troops to supply its bases any more. Now US troops have to come out and defend the convoys.
I don't know if the US is going to try to take and hold Iranian territory, but if it does, it will find a dug-in prepared and resource-rich insurgency pre-positioned that will immediately surpass the worst insurgency-based fighting the US has faced so far in Iraq. If not, US activity in Iran will be limited to air strikes. Air strikes did not remove Hussein from power, did not remove Hezbollah from power, didn't even remove Hamas from power.
All air strikes accomplish practically is build an image of a cowardly armed forces that are only effective when targeting civilians and civilian infrastructure. The US will get some hospitals, some power plants, some water-purification plants but these will outrage the world. There will not be direct consequences for this outrage the colonies of Egypt, Jordan or Saudi Arabia, but Turkey will be under tremendous pressure to end its alliance with the United States and Israel, possibly irretrievably. That will be the most important immediate diplomatic consequence, but the rest of the free Muslim world will turn against the US in ways that will be harmful later.
There may or may not be attacks on mainland US targets. There very likely will be attacks on Western European allies of the US. These will not be decisive, but will cause the citizens of Western countries to feel some discomfort from this war. I honestly do not think Hamas or Hezbollah would need to be activated against Israel, even if it was Israel that launched the first attack because Iran can attack the US directly more effectively while holding Hamas and Hezbollah capabilities in reserve, so that they can continue to deter Israeli attacks on their territories.
Eventually the situation will stabilize with the US taking constant casualties in defending the supply lines to its forces in Iraq and/or Afghanistan while Iran is under US bombing but not nearly enough to displace the regime. With direct Iranian involvement I think it would be conservative to project a death rate for US troops of twice the peak death rate from the Iraq occupation.
Iran already produces surface to air missiles, but not the most advanced in the world. On the other hand, Russia produces state of the art shoulder fired anti-air missiles. Russian "organized crime" groups could "smuggle" weapons to the Iranians, just because the longer the US remains in a losing stalemate in Iraq and Afghanistan, the less able it is to influence Russia's regions of interest. Hillary Clinton can complain to the Russians about the "smuggling" but the Russian reply would be "or what?".
Once firing starts, Russia wants and likely can achieve a reversal of World War II. The US post war advantage over Europe and Russia, that has been eroding since that war, can be fully dissolved before the US gets out of Iraq and Afghanistan. If Iran needs help to cause this to happen, it has good internal supply lines, connections and resources to do so. Russia also has its own nuclear deterrent. The US has no leverage to prevent Russia from draining it the way the US did when the USSR invaded Afghanistan. This means Russia has an interest in preventing Iran from running out of material to continue its war effort and may well do so.
We have not talked about the Persian Gulf. At some point, shipping is likely to stop because tankers, one way or another, will be destroyed as they try to pass. More importantly though, the US Navy has not come under serious fire in a long time and how it fares under fire is important information that both the Russians and Chinese have an interest in acquiring. If Chinese "triads" "smuggle" anti-ship weapons to Iran, they will be able to experiment on active US Naval defense systems that will provide data for Chinese planners of contingencies involving Taiwan and designers of future weapons systems. Possibly an attack will get lucky and cause huge financial damage and hundreds or thousands of US troop deaths. Clinton will complain but China's reply will be "or what?" The US does not have a credible response to unprovable Chinese activity while it is already engaged in a war with Iran.
In Iraq, the US has to decide to continue trying to supply the troops, or fight their way out of the country. Either way, the US is no longer effectively able to influence Iraq's government or armed forces and Iraq will now be a fully Iran-friendly state bordering Saudi Arabia and Jordan. Israel is committed to defending Jordan and while there will not be a set-piece invasion, Iranian and Iraqi teams will be able to enter nearly at will and apply pressure to the Jordanian colonial dictatorship.
Lastly we have not talked about Iran's nuclear program. Iran certainly will direct all of the country's scientific resources toward building a weapon. Pakistan, outraged by the US' attacks on Iranian civilians will be difficult to deter from assisting in any way they can. It is very unlikely that Iran has not built a weapon two years after the shooting started. It will not use the weapon in this conflict, but once it has it, the situation on the ground has changed permanently. Iran will not give up the weapon once it has it under any circumstances, under any government even if in the distant future somehow the US gets regime change somehow.
Iran will be bombed the way Lebanon, Iraq and Gaza were bombed. Unlike Gaza, the US will not be able to prevent rebuilding. The US will face, conservatively, 200 US deaths a month until US voters decide they've had enough. Leaving means either suing for peace and paying reparations to Iran or the soldiers evacuating under fire, leaving their material behind. Once the US is gone, its positions in Afghanistan and Iraq will be gone with it. It will also have lost Turkey and the colonies in Jordan and Saudi Arabia will be under more pressure than they've ever been under.
Iran will rebuild, be fully nuclear and by the calculations of the Iranians, the war can be expected to produce both national cohesion and in some engineering schools and seminaries will be Khomeinis and Ahmadinejads for who have been radicalized and taught the importance of ideological sacrifice preparing them as leaders of future generations. With the US having no further appetite to intervene in the Middle East, it will be relatively soon that Iran will regain its position as the dominant power of the region.
The US ability to project force into the Middle East will be permanently diminished and US financing of this expanded war will accelerate the US' decline as an economic power. How did this war start again? The US didn't want Iran to have the capability, in theory, the make a nuclear weapon because that would mean Israel would lose what it considers its necessary ability to threaten its neighbors? It is very likely that a generation of Americans further removed from WWII will reconsider the US commitment to Israel after this war.
Russia will be the undisputed winner as it benefits not only from the US losing position relatively and the US' inability to concentrate on Europe or other objectives while fighting Iran, but it will also benefit from the spike in energy prices. Russia may lock in its gains with a nice color revolution in Poland just to complete the symmetry of historical justice being done for US intervention in its previous invasion of Afghanistan.
I'm pretty sure a war with Iran would be the worst mistake in the history of the United States and would set in motion a process that would result in the US being no more globally relevant than Brazil. I'm also pretty sure US strategists are aware of the risks hostilities escalating to war with Iran entail. I likely got some details wrong, I also likely missed some problems a war would cause for the US. I'm fairly confident that a decade after the end of the war, it will be generally acknowledged that between Iran and the US, Iran ended up being the victor.