How did the invasion of Irak work out for the US (I won't even ask how it worked out for the Irakis)? And that was done with powerful forces staging over many months and building up from border countries with their assent. Who is going to allow the US to stage a land invasion of Iran from their territory?
That leaves invasion from the sea: The US marines operate Expeditionary units organized around twelve groups of three ships each carrying a force of about 5000 each marines - but that includes the air groups and support, so it's closer to 3500 ground troops. If all twelve groups are available (doubtful, especially since one of the cornerstone one burned to a crisp and has yet to be replaced and otherwise, some are in refit) that's about 42,000 troops. The US Army can probably put a further 3,000 to 4,000 troops there, so lets say a total of 45,000 boots on the ground, light forces with limited armour and guns and then, no follow on force until the ships can return to wherever they would pick up the next load up.
Anybody here thinks they can take a country of 95 million inhabitant, whit a very large number of them armed and with local knowledge of difficult terrain?