This analysis of an assault on Iran is from prior to the New Year dust up and the death of Soleimani and his Iraqi militia commander buddy. But the reality is that force estimates haven’t changed. Capabilities haven’t changed. The current on hand military forces haven’t changed.
War with Iran, realistically, is one sided. The United States forces aren’t limited in their capacity to win these fights, just by what casualties would be taken in the process. Even with a united Iran resisting on the magnitude of Imperial Japan the United States could win this fight. But unity within Iran and under the regime is nowhere near the levels seen amongst the Imperial Japanese. Despite the control exercised by the government of Iran, there is widespread dissatisfaction that has flared in protests in recent history. The likelihood of Iran devolving into disparate factions during a ground war and kicking off another insurgency problem is the most problematic.
Factions would be grabbing up their own little micro empires and exploiting offered resources from the U.S. and coalition forces. Each neighboring nation would want to exert their influence on a restructured Iran, just like Iran has done with Iraq.
In short. A short, violent, devastating fight where we leave at the end offers us the best chance for low casualties. Staying within Iran after poking holes and making a power vacuum is where the risk increases. This wouldn’t be a casualty free fight, but it would be heavily one sided.