The belief that the Iraq war was a monumental blunder is now widely held. In the public mind, the decision to dethrone Saddam Hussein has also become a ready explanation for nearly every ill to befall the world since. The latest baleful effect attributed to regime change in Baghdad is the ascent of the Islamic State in Iraq and Syria (ISIS). No less a figure than President Obama lists ISIS as one of the "unintended consequences" of the Iraq war, which evidently imparts the lesson that the U.S. should "aim before we shoot."

A critical observer might note that this argument suffers from one conspicuous drawback: the roots of ISIS lay not only in Iraq, where America intervened, but also in Syria, where it did not.

This is not the place for an exhaustive recapitulation of ISIS’s evolution, but a brief sketch will show that it rose first in post-Saddam Iraq, and was defeated there, before it was resurrected in the present chaos of Iraq and Syria.

After the U.S. invasion of Iraq, Osama bin Laden established al-Qaeda in Iraq and designated Abu Musab al-Zarqawi to be its "emir." Zarqawi proceeded to drench Iraq in blood - Iraqi blood - in the hopes of sparking a sectarian war between Sunni and Shia. This grisly strategy put Iraqi society on a knife's-edge, but it also opened an ideological fissure between bin Laden and Zarqawi that would eventually lead jihadists associated with the latter - who was eliminated by a U.S. airstrike in June 2006 - to form ISIS.

Before long, the U.S. effort in Iraq was salvaged by a combination of counterinsurgency strategy (the "surge") and the Anbar Awakening. Zarqawi's force was overwhelmed, dispossessed of tribal support in the Sunni hinterland and decisively routed on the battlefield. Although Zarqawi's scheme to restore a lost Islamic empire lay dormant, it did not become extinct. In 2011, two events conspired to give it a new lease on life.

The first came in March, when democratic demonstrations broke out on the streets of Damascus. The peaceful protests were ruthlessly crushed by Syria's calcified dictatorship, which ignited an armed rebellion. In response, the Assad regime pulverized entire neighborhoods by bullet, bomb and gas. The decision in Western capitals to withhold military aid from secular and nationalist factions of the Syrian revolt allowed jihadist bands - who were blessed with less diffident foreign allies - to become the most lethal contingent of the opposition.

The second event that led to ISIS's resurgence came in December 2011, when Iraq's Sunni "Awakening" unraveled thanks to a lethal mixture of American and Iraqi mistakes. America's indifference to Iraq's fate led to a categorical troop withdrawal that exacerbated Prime Minister Maliki's sectarian instincts. He not only demobilized and harassed the Sons of Iraq that had vanquished al-Qaeda in Iraq (AQI), but also labored to turn the Iraqi army into a gigantic Shia militia. Once more, the conditions were set for Sunni power to respond, this time in the form of jihadist violence.

In the midst of one-dimensional debates over post-Iraq U.S. foreign policy, what is lost is the brutal fact that inaction is a policy with unintended consequences of its own.