Whether the US has any military options against Iran, short of brandishing its nuclear weapons, depends on whether Iran is scared enough (or divided enough) to prevent it from adequately responding to any US attacks. Otherwise, the US has no realistic military options against Iran and the US military knows it as well.
No one in the US is even considering a ground invasion aimed at conquering the country. To be in position to do so, the US would need to institute the draft and do a lot of other things it is in no position to do right now.
Air strikes are easier said than done. While the US is more than capable of launching limited air strikes against a limited number of Iranian facilities, a more comprehensive target list would require various platforms and basing arrangments. Aircraft carrier in the Persian Gulf would be vulnerable and indeed one of the best signs the US might be up to something is if it pulls out its carriers out of the Persian Gulf. However, the more those carriers move away from Iranian targets, the fewer targets they can hit, especially in the initial days of a conflict where refueling aircraft would themselves be potentially vunerable. Some of the targets the US would want to attack are well defended and the US will lose some aircraft in the process. Others are largely invunerable to conventional bombs, protected by deep bunkers.
Once US operations against Iran begin, Iran will be able to launch retaliatory strikes and begin operations in several different fronts. I will focus here on a couple of them.
In the Persian Gulf, Iran will try to close the straits of Hormuz, sink a couple of tankers there to block the passage-way, mine the surrounding waters, and use its anti ship missile capablity to stop oil traffic from the region. Simultaneously, having blocked the Staits of Hormuz, Iran will have also managed to trap the US navy in those waters. Doing so, Iran will try to use swarming techniques to overwhelm the defenses of the US warships, while it be using its substantial anti ship missile capability to attack US naval vessels. US war games have shown Iran could succeed in delivering major blows to the US navy in the process.
At the same time, Iranian ground to ground missiles will be launched against US bases in places such as Qatar, Bahrain et al. Iranian strike aircraft will take part as well, although those would be vulnerable and Iran could suffer heavy losses relying too much on its air force.
As these battles wage on, Iran would certainly try to cut off the supply lines to US troops in Iraq and Afghanistan, while bombarding those troops through its ground to ground missiles. Iranian commandos operating in smaller units will also enter Iraq and team up with other Iranian assets to pick out the US forces in Iraq and Afghanistan.
There are other levers and other actions Iran could take to exacerbate the US position, and absent a ground invasion, time will be on Iran's side. The US would then be left with three options: accept defeat, threaten nuclear war, or institute the draft and prepare for a land invasion of Iran.
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