The same scenario played out in Vietnam. The US could never succeed because:
- the enemy was intermingled with the "friendly" civilians, and they couldn't be told apart, leading to everyone being treated brutally and potential friends becoming enemies
- the enemy was prepared to fight to the death, for years if need be, and knew they could outlast US public opinion
- the enemy knew they could prevail because of centuries of history defeating much larger opponents (in Vietnam's case, of them previously defeating France and China).
All of these same conditions would be present in a ground war in Iran, with some religious fanaticism thrown in on top.