A volunteer who is going to join the Iraqi army to fight against the predominantly Sunni militants who have taken over Mosul and other northern provinces, reacts to camera as he waits to register in Diwaniya province on June 12, 2014. Since Tuesday, black clad Islamic State in Iraq and the Levant (ISIL) fighters have seized Iraq's second biggest city Mosul and Tikrit, home town of former dictator Saddam Hussein, as well as other towns and cities north of Baghdad. They continued their lightning advance on Thursday, moving into towns just an hour's drive from the capital. Reuters/Alaa Al-Marjani

The Islamic State of Iraq and Syria, a radical Islamist organization even al Qaeda shuns for being too extreme, now controls the northwestern third of Iraq, having captured the cities of Mosul and Tikrit, and is advancing toward Baghdad. ISIS has a centralized bureaucracy, a large, disciplined militia and an ability to sell oil on international markets. In effect, the group has formed the basis of a new, radical Sunni state -- one that has even rendered the official border between Iraq and Syria obsolete.

For President Barack Obama, whose opposition to American involvement in Iraq helped define his national political career, Iraq has re-emerged as a major headache, a country whose violent divisions he has been unable to escape. Obama based his campaign for president in 2008 on the fact that he, unlike primary opponent Hillary Clinton or Republican John McCain, had understood from the beginning that the Iraq War was a mistake. In a speech he delivered that March in Fayetteville, North Carolina, then-Sen. Obama said “the central front in the war against terror is not Iraq, and it never was.”

Six and a half years later, this statement is no longer true.

For the Obama administration, which withdrew the last U.S. forces from Iraq in December 2011, the emergence of ISIS marks a significant setback. Iraq is not “sovereign, stable, and self-reliant” as the president claimed it was then, and the state now appears on the verge of complete collapse.

What accounts for this failure? First, it is important to examine the role of Nouri Kamal al-Maliki, who has served as Iraq’s prime minister since 2005. During his tenure al-Maliki, a Shi'a Muslim, pledged to include more members of Iraq’s Sunni minority into the government and military, providing them with a stake in the viability of the Iraqi state. But he has largely failed to do this, leaving a marginalized and resentful Sunni population susceptible to the lure of radical groups. This decision undermined a central goal of the U.S. mission in Iraq.

“It’s so easy to blame everything on the U.S.,” said Judith S. Yaphe, a Middle East scholar at National Defense University. “But the Americans tried to insist on integration, and the Iraqis resisted.”

The situation in neighboring Syria has also had a major spillover effect. Originally a civil conflict pitting President Bashar al-Assad against an array of domestic forces, the Syrian war is now a regional issue, one that has upended national divisions. ISIS, an offshoot of al Qaeda in Iraq, a radical Sunni militia headed by Abu Musab al-Zarqawi until his death in 2006, stepped into the vacuum caused by Syria’s disintegration, recruiting and training militants from around the world. Having grown in size and sophistication, ISIS crossed into Iraq and easily routed Iraqi forces, which dissolved despite being well-equipped with American weapons. So the Sunni insurgency that raged across Iraq in 2005 and 2006 has effectively now merged with one in neighboring Syria.

Whether or not his policies contributed to the current mess in Iraq, it is nonetheless Obama’s responsibility to craft an American response. Maliki has asked Washington to come to his aid with air strikes, but Obama has so far refused -- the president has merely said on Thursday that his administration is “reviewing options.” This deliberation has frustrated his critics, who have accused the administration of lacking direction.

But the biggest obstacle to intervention in Iraq may be public apathy in the United States. Exhaustion from almost 13 years of war in Afghanistan, combined with presistent economic concerns, have led Americans to turn inward -- a stark contrast from the public mood surrounding President George W. Bush’s invasion in 2003.

Nevertheless, Obama’s triumphant withdrawal from Iraq -- an accomplishment he trumpeted during his successful re-election effort in 2012 -- now appears, more than ever, to have been premature.