Did Iran Attack American Troops In Iraq?

CNN reports that American military investigators believe the January 20th attack on a military compound that killed five US soldiers may have either been conducted by Iran or by Iranian-run insurgents. The level of sophistication in the attack, conducted by terrorists in American military uniforms, showed too much sophistication to have originated from one of the native insurgencies:

The Pentagon is investigating whether a recent attack on a military compound in Karbala was carried out by Iranians or Iranian-trained operatives, two officials from separate U.S. government agencies said.
“People are looking at it seriously,” one of the officials said.
That official added the Iranian connection was a leading theory in the investigation into the January 20 attack that killed five soldiers.
The second official said: “We believe it’s possible the executors of the attack were Iranian or Iranian-trained.”
Five U.S. soldiers were killed in the sophisticated attack by men wearing U.S.-style uniforms, according to U.S. military reports.

The investigation just started, and the Pentagon will probably look at a number of possibilities for the attack. However, given the description of the attack and its effectiveness, it seems a little over the pay grade of even the Ba’athist remnants. Since this occurred in Karbala, a predominantly Shi’ite area, Sunni insurgents and al-Qaeda also seem unlikely suspects.
Earlier on Tuesday, Time Magazine reported that Iran has a motive to attack Americans in Iraq. The Revolutionary Guard wants some measure of revenge for the capture of five Iranians in Irbil, at least some of whom belong to the IRGC. Time speculates that the IRGC wanted to send a message, and that the number of casualties were specifically selected to make sure that no one misunderstood it.
What happens if the US concludes that Iran did indeed conduct this mission against American servicemen? It would be an act of war, although the presence of Iranian Revolutionary Guard soldiers in support of insurgents also qualifies. The Bush administration might be tempted to retaliate with some air strikes, perhaps selected especially for the nuclear program Iran seems keen to pursue at all costs. However, one can imagine the outcry that would cause, not just among our European allies but also leading Democrats in Congress. It would not take long for at least a few of them — Maurice Hinchey springs to mind — to accuse the Bush administration of manufacturing the evidence pointing to Iran in order to justify an attack on that nation.
If the evidence points in that direction, there will be no big rush to respond. It might do some good to make the Iranians sweat for a short period. However, Bush will have to confer with the Democrats and make it clear what happened, and impress upon them the need for serious action to deter the Iranians from attacking Americans in the future. We’ve let too many of these incidents pass without consequence to the mullahs, and every unanswered insult begets more of the same.
UPDATE: Bill Roggio analyzed this last Friday and came up with the same answer.