The United States military is, for one thing, in much worse shape today than it was in March 2003 with far fewer resources at its disposal (see the Iraq War). The Iranian military, meanwhile, is in better shape than Iraq’s army was, since it hasn’t been subjected to more than a decade of stifling sanctions. Iran is geographically larger than Iraq. Its population is about twice as large as Iraq’s. Perhaps more to the point, the vast majority of the trouble in Iraq has been made by a distinct minority of the population -- the one Iraqi in five, more or less, who is Sunni Arab, the dominant group in the Baathist ancient regime. Fully half of Iranians are Shiite Persians, so we’re talking about a nationalist backlash with a population base about four or five times as large as the one we're facing in Iraq.
So, after more than half a century of active meddling—protecting our interests, promoting our values, encouraging democracy, fighting terrorism, seeking stability, defending human rights, pushing peace—it's come to this. In Iraq we find ourselves unwilling regents of a society splitting into a gangland of warring militias and death squads, with our side (labeled "the government") outperforming the other side (labeled "the terrorists") in both the quantity and gruesome quality of its daily atrocities. In Iran, an irrational government that hates us with special passion is closer to getting the bomb than Iraq—the country we went to war with to keep from getting the bomb—ever was.
I'm truly astonished that we're actually discussing another war, when we haven't finished the last one -- and that one against a weaker foe.
I suspect that, a hundred years from now, someone will pen the definitive history of our time, and the title of that book will be Batshit Crazy.