For once, the US are losing in Iraq. Even GWB admitted to that this week.
Winning a guerilla war / war against an armed insurgency with conventional armed forces is just not going to work out. Basically, you are asking the Marines to do police work i.e. act as counter-terrorism units.
You are using a broad sword when you should be using a scalpel (sp?).
It's a stalemate at best, and whoever said the US were winning because "we kill more of them than they kill of us" is a moron, I am sorry. You are exchanging body bags for little to no considerable gain.
You have removed Saddam, and offered the Iraqis a chance at democracy. Now it's time to leave and let the Iraqis figure out themselves if they are ready for the challenge.
Now, with regard to the topic at hand, I think a war against Iran would be difficult to realize for three reasons:
1. what would be the justification ?
2. Does the US even have resources to pull it of alone ?
3. What are the implications for the region with regard to stability ? In other words, what are the long-term strategic goals behind such an idea ?
1. With Iran being in no violation of International Laws or treaties, this would be a pre-emptive strike, with no formal justification. And even if Iran would be violating any such agreement, law or treaty, a full-scaled invasion would be totally disproportionate and uncalled for.
2. No. Look at the budget, look at the manpower. Gen. Abizaid said it himself. No way. Iran is fucking huge, 70 Million people. And although not all of them support their crazy President, they'd most likely rally behind him under such circumstances. Also, Iran would probably receive aid from other islamic nations such as Syria.
At the same time, international support for the US is at an all-time low.
3. Well, if you think Iraq is going badly, imagine the same fuck-up in Iran. And multiply it by ten. Insurgency, guerilla war, IED's, you'd have all of that all over again. Result: region will be even more de-stabilized, with growing terrorist activities. Also, the prospect of having to place an occupation force in Iran for years to come doesn't look so good, does it?
Finally, as far as I know, the american public is growing more and more war-tired. I don't think you'll have widespread support for a campaign against Iran among regular americans, who most likely don't want to see their sons, daughters, fathers and brothers come home in a body bag any longer. It has been going on for too long already, without much result.