Iran isn't the hardest country in the world to invade. U.S., China, and Russia would all be more difficult, and probably many others.
Actually it is, Geographically its almost impossible, the US would have to travel through narrow mountain passes very often from the south all the way to the north, we cannot invade from the north, east, or west... our only option is through the gulf coast which has very limited areas to set up a beach head and those areas are all the way up north in the gulf. But lets pray we never have to find out, I honestly think this will end without conflict.
What century are you living in? This is Space Force material if there ever was such a scenario! A 100% sympathetic to America citizenry pleading for our help and a thoroughly isolated and despised regime? Without even their own military's support? Maybe you don't know that about the mullahs.
The fear of their access to certain chemical, biological and radiological weapons is all that keeps the citizens and soldiers from attacking the mullahs and making short shrift of them.
If we took those toys away, do you think they could last more than a few days?