To clarify Clark's comments above, here is another quote from that interview back in 2007. Clark was not openly advocating for an invasion of Iran at the time.
GEN. WESLEY CLARK: Well, starting with Iraq, then Syria and Lebanon, then Libya, then Somalia and Sudan, and back to Iran. So when you look at Iran, you say, “Is it a replay?” It’s not exactly a replay. But here’s the truth: that Iran, from the beginning, has seen that the presence of the United States in Iraq was a threat — a blessing, because we took out Saddam Hussein and the Baathists. They couldn’t handle them. We took care of it for them. But also a threat, because they knew that they were next on the hit list. And so, of course, they got engaged. They lost a million people during the war with Iraq, and they’ve got a long and unprotectable, unsecurable border. So it was in their vital interest to be deeply involved inside Iraq. They tolerated our attacks on the Baathists. They were happy we captured Saddam Hussein.
But they’re building up their own network of influence, and to cement it, they occasionally give some military assistance and training and advice, either directly or indirectly, to both the insurgents and to the militias. And in that sense, it’s not exactly parallel, because there has been, I believe, continuous Iranian engagement, some of it legitimate, some of it illegitimate. I mean, you can hardly fault Iran because they’re offering to do eye operations for Iraqis who need medical attention. That’s not an offense that you can go to war over, perhaps. But it is an effort to gain influence.
And the administration has stubbornly refused to talk with Iran about their perception, in part because they don’t want to pay the price with their domestic — our US domestic political base, the rightwing base, but also because they don’t want to legitimate a government that they’ve been trying to overthrow. If you were Iran, you’d probably believe that you were mostly already at war with the United States anyway, since we’ve asserted that their government needs regime change, and we’ve asked congress to appropriate $75 million to do it, and we are supporting terrorist groups, apparently, who are infiltrating and blowing up things inside Iraq — Iran. And if we’re not doing it, let’s put it this way: we’re probably cognizant of it and encouraging it. So it’s not surprising that we’re moving to a point of confrontation and crisis with Iran.
My point on this is not that the Iranians are good guys — they’re not — but that you shouldn’t use force, except as a last, last, last resort. There is a military option, but it’s a bad one.