Mass* Occupation wouldn't be in the military's strategy. They only need to crush key military strongholds and disrupt the flow of government. There are so many ways to do this to Iran its silly to think you need to pack millions of troops on a border and march. Its archaic.
That's step 1, remove the Iranian government. That's the easy part. Step 2 is actual occupation. How do you plan on fighting an insurgency many times larger, better equipped(and likely to be funded and equipped by Russians and Syrians), and many times better organized? The US struggled heavily in Iraq, 5,000 killed, 32,000+ wounded. Iran would inflict casualties not seen since Vietnam.
Looking at the Iraq war, we went in with a coalition of about 600,000 troops.. You're going to need twice that for Iran. You can guarantee the US wont recieve any help this time either. The US would be pulling its entire army,marines, and many reservists just for the invasion. Occupation of Iraq took 170,000 troops at its height, so, it's probably going to take at least 300,000+ to occupy Iran. Once the casualties start rolling in and deployments needing to be rotated, how exactly are they going to be able to maintain troops levels without a draft? The manpower simply isn't there.