If the dipshits actually dare and go through with it, then we're gonna witness US KIA levels not seen since WW2, entire units will be gutted and the US military could experience an unprecedented morale failure
Iran for the most part will be fine, most of the urban centers are far to the north of anywhere the Americans could conceivably approach logistically, so the ground invasion will be defined by Iranian drone swarms decimating American units bogged down on some sandy shore or hilly outcrop
I just don't see how the US military can absorb mass casualties of the scale an invasion would cost, considering how universally terrible the morale and mental health of the US military is even in normal conditions