As a radical Sunni insurgency imperils Iraq, the United States and Iran have finally stumbled into a common enemy. So what?

The situation

Despite 35 years of hostility, enmity, proxy war, etc., the swift takeover of wide swaths of Iraq by the terrorist group the Islamic State of Iraq and Syria (ISIS) has suddenly endowed the United States and Iran with a shared interest.

The Iranian government sees the advances made by the al Qaeda-inspired Sunni group ISIS as a threat to Iraqi Prime Minister (and fellow Shiite) Nouri al-Maliki. The United States, which spilled blood and untold treasure upending Iraq and then working to stabilize it as well as training the Iraqi army, doesn't want to see its work entirely undone by marauding extremists.

So, quite suddenly, America and Iran want the same thing for the first time in seemingly decades. "The enemy of my enemy" maxim is trending again.  Does this mean there's an opening here?

The short and long answers

They're both "no."

Earlier today Iranian President Hassan Rouhani signaled an unlikely willingness to conduct joint actions with the United States to help stabilize Iraq, you know, if it ever comes to that. When asked if Iran could cooperate with the United States on Iraq, here's what he said.

We have not seen the US do anything for now. Any time the Americans start to take action against terrorist groups, we can consider that."

This followed yesterday's quickly maligned State Department utterance:

Great! The United States and Iran will work together on Iraq and then forge ahead, using their cooperation as a means to make progress on the stalled nuclear talks. And then, in the sake of regional peace, Iran will back off its assistance of Hezbollah, undeclare its desire to see bad things happen to Israel, and distance itself from Russia and Syrian dictator Bashar Assad. Emboldened by the security advancement, the United States will be able to convince Israelis and Palestinians to make peace and everything else will finally quiet down. 

Not even that (reductive) fantasy will see the light of day. The history is too dark, especially on the topic of Iraq. As the New Yorker's Robin Wright reminded us in a post titled...."The Enemy of My Enemy":

Iran was America’s nemesis throughout the eight-year American war in Iraq. American officials regularly berated the Iranians for providing arms (including I.E.D.s) and strategic guidance to Iraqi militias. “They are responsible for providing the weapons, the training, the funding and, in some cases, the direction for operations that have indeed killed U.S. soldiers,” General David Petraeus told reporters in 2007."

Here's another way to put it:

Should the two countries manage to work together on anything, it'll be a miracle. Should they work together on Iraq, it will be a temporary alliance of total convenience.