Iran Is A Tougher Enemy Than Iraq

There was a saying in the Bush administration prior to the 2003 Iraq invasion: Everyone wants to take Baghdad but only real men want to take Tehran.
Baghdad was always the easier target. Iran is a different problem entirely.
Iraq in 2003 had a broken military, a population traumatized by sanctions and internal repression, and no serious geography to defend. The country sits on a flat alluvial plain. You could drive from Kuwait to Baghdad in days, and the Americans did. The air defenses were obsolete, the command structure was hollowed out, and large parts of the army simply walked away.
Iran is none of those things. It has three times the population, roughly the geographic size of Western Europe, serious mountain ranges along its western and northern borders, and a government that has spent forty years preparing for exactly this scenario. The Revolutionary Guards exist partly as a conventional fighting force and partly as an ideological army designed to keep fighting even if the regular military collapses. The doctrine is explicitly built around the idea that Iran cannot match American airpower, so it will make the cost of occupation or regime change unbearable through asymmetric pressure, proxy networks, and yes, Hormuz.
The saying also reflects something the Bush administration understood intellectually but chose to ignore in practice. Taking Baghdad was the easy part. Governing it was the catastrophe. Tehran would not even get to the governance phase before the problems started. Any serious ground campaign against Iran would face supply lines that stretch through some of the most difficult terrain in the Middle East, an enemy with genuine popular nationalism behind it, and a strategic depth that Iraq simply never had.
No serious military planner wanted to take Tehran. The line was less a boast than a warning dressed up as bravado.