Jihadi extremists and Sunni tribes have commandeered north Iraq and are fighting Iran-backed Shia militiamen and volunteers within 50 miles of Baghdad.
The conflict has all of the makings of a sectarian war at a time when the civil war in neighboring Syria grinds on in its fourth year. Brookings expert Matthew Pollack thinks that, absent a huge influx of military assistance from an foreign backer, "the fighting will probably continue for years and hundreds of thousands will die."
Iran's primary interest involves protecting the Shia crescent, which comprises Shia populations from Iran to to the Mediterranean coast in Lebanon.
That's why Tehran has been training Iraqi militias for years while infiltrating the Shi'ite-led government of Iraqi Prime Minister Nouri al-Maliki.
The above map from Morgan Stanley illustrates why Iraq is crucial to the Middle East amid flaring sectarian tensions. Iran is 91% Shia, Iraq is 66% Shia, Syria is 16% Shia, and Lebanon is 30% Shia.
Assad and Hezbollah are battling a overwhelming Sunni majority in Syria, and Tehran needs to maintain a robust supply line from Iran to Damascus and Beirut. Doing so requires fighting the Sunni minority that's revolting in Iraq.
Basically, Iran is angling to dominate the regions as it goes up in flames. Proactive moves in Syria and Lebanon - combined with U.S. capitulation - have worked out to Tehran's advantage.
Consequently, Iran is moving fast to bolster Baghdad and are willing to accept any help the U.S. is willing to give it.
Morgan Stanley