[w]e would have a credible and effective military option to threaten Iran with that wouldn't just leave them stronger and tougher and better positioned.

How long do you go on "threatening" military action before you have to cough up? Saddam was convinced, even after Afghanistan, that we would never dare invade them. Why would Iran behave any differently if we had parked troops in neighboring nations (which nations would those have been, by the way)? And are you conceding that maybe invading Iran would have been acceptable? How many other liberals might feel that way? Keep in mind that much of the Left doesn't even think we should have gone into Afghanistan.

Also keep in mind there's a lot to our military besides ground troops and tanks. Our enemies need to keep that in mind.

We would have allies that could play active coordinated roles in carrot-and-stick talks.

Who? Who would go in with us in threatening Iran in the United Nations that didn't go in with us in Iraq? France? Germany? Russia?

Do you have any evidence at all that their opinions of our foreign policy, or their opposition in the U.N., would change by simply changing "Iraq" to "Iran?"

...but with a Big Bad USA occupying Iraq for reasons that turned out to be patently false, looking like a imperialist power, nobody is going to be buying what the Secularists are selling anytime soon.

Your premise that Iran would have drifted into democracy and tolerance if we'd left Saddam alone is interesting. Maybe. Maybe not. How would Iran feel about an occupied nation on their other border?

Again, the ground assumption for statements like this is that somehow our taking out the government of Afghanistan and decimating al Qaeda was not going to irritate the Middle East like invading Iraq would. Given that Afghanistan was the heart of radical Islam, and that we have constantly been told that Saddam and the Islamists were mortal enemies, I have not seen anyone at all explain this odd premise to my satisfaction.

It's the details that make the sunny alternative scenario start to cloud up.