The stunning success of the Islamic State of Iraq and the Levant (ISIS) in taking and holding Mosul, Iraq's third largest city, and apparently taking Saddam Hussein's hometown of Tikrit today, has done far more than lay bare the striking deficiencies of Iraq's central government and its security forces. The ISIS advance is a culmination of disastrous choices both inside Iraq and out.

The US argument that Saddam Hussein was in cahoots with Al Qaeda (he wasn't) led to an invasion that spurred the development of the strongest core of Sunni jihadists in the Middle East. Syria's Bashar al-Assad, threatened by the Bush administration that he was next on the regime change list, tolerated Syria's borders with Iraq becoming a transit zone for jihadis eager to join the Sunni-based insurgency, reasoning that any potential blowback was worth tying down the powerful US military.

Iran, a member of Bush's "axis of evil," armed and supported Shiite militias with much the same reasoning as Mr. Assad for tolerating the jihadists. Saudi Arabia, alarmed at the Shiite-Islamist government the US helped install in Iraq – sure to develop warm ties with Tehran – tolerated a flood of financing to Sunni jihadis from wealthy donors in hopes of containing the influence of Iran.

All that helped set the stage for Iraq's sectarian civil war, which claimed well over 100,000 lives and saw traditionally mixed towns and neighborhoods sorted into exclusively Sunni or Shiite enclaves. And now, thanks to the Syrian civil war and the ineptitude and chauvinism of Iraqi leaders like Prime Minister Nouri al-Maliki, a jihadi army devoted to harsh Islamic law and the subjugation of religious minorities and women is rampaging freely across large swaths of Iraq.

And suddenly, old rivals are realizing they have something in common. Iran has warm ties with both Baghdad and Damascus, and has been heavily funding Bashar al-Assad's fight for survival against a rebellion composed of both jihadists and more secular-leaning groups. The US, which spent over $2 trillion on a war it hoped would transform Iraq into a stable democracy and a reliable ally, is just as worried about the threat to Baghdad posed by the jihadists as Iran is. Syria's Assad is practically pointing and jeering "I told you so" from the sidelines, though some analysts believe that if President Obama had been willing to intervene in the early months of Syria's war, more palatable rebel groups would have prevented groups like ISIS from gaining steam.

The Saudis, too, appear to be ruing their choices. In February, King Abdullah abruptly dismissed spy chief Bandar bin Sultan, who was the point man for the kingdom's efforts in Saudi Arabia. Prince Bandar, at one-time referred to as "Bandar Bush" because of his close relationship with former President George W. Bush, had been funneling arms and cash to jihadist groups like ISIS in an effort to topple Assad.

The support to jihadists infuriated the Obama administration, and eventually unsettled senior Saudi figures, well aware of how much damage Al Qaeda militants had done inside the country in the 1990s and early 2000s. Bandar was replaced with Prince Mohamed bin Nayef, who led the fight against Al Qaeda in the country and narrowly survived an assassination attempt by a suicide bomber in 2009.

Presumably, the Assad regime and people like Bandar imagined they could control the jihadi groups. But they are finding out instead that they've helped create something like Frankenstein's monster. And ISIS is not alone – other Iraq-based jihadist groups appear to be joining the fray. As for what's called "core Al Qaeda," the Pakistan-based group led by Osama bin Laden's successor Ayman al-Zawahiri, they have little influence or control either. Al Qaeda publicly chided ISIS for its abuses of Syrian civilians earlier this year, and ISIS' response was basically "get lost."

What is the US to do now? The US delivered about 300 Hellfire missiles, helicopters, and light arms to Iraq earlier this year, when ISIS-affiliated groups were overrunning the towns of Fallujah and Ramadi in Anbar province. That was on top of the billions of dollars spent on arming and equipping the Iraqi army and police from 2003 to 2013. But the cops and soldiers in Mosul largely fled, leaving behind weapons, ammunition, and US-built armored humvees. ISIS members have been parading around with this US-supplied equipment for the past few days.

The International Organization for Migration estimates that 500,000 people have fled from Mosul – about a third of the city's population – and there have been steady reports out of Iraq of ISIS making advances on both Tikrit and the key oil refining town of Bayji, which is about halfway to Baghdad from Mosul.

Will Maliki seek US help – perhaps air support? He could. But that would leave the US in an interesting position, putting it on the side of not just Baghdad, but the Syrian regime and Tehran.

Meanwhile Iraq's Kurds, whose peshmerga fighters based just north and east of Mosul in autonomous Kurdistan and are probably the country's most capable force, are reportedly pushing Maliki for concessions in an oil revenue dispute in exchange for helping the central government retake Mosul.

For now, the Kurds are holding the line in Mosul's east and providing safety to those fleeing the chaos.

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The US is at a loss, expressing "deep concern," vowing to continue to support Maliki's government much as it has, and calling for the same sectarian and political reconciliation in Iraq that US government's have been ineffectually calling for since at least 2004.

There aren't any easy answers. But Iraq's crisis is making for strange bedfellows, with both the US, Iraq, Iran, Syria, and Saudi Arabia eager to see the growing strength of ISIS curtailed. A kernel of a solution might be found in this common interest.