The U.S. left behind an Iraqi army largely trained and armed by the U.S. and a country mostly at peace at the expense of 4,480 dead U.S. soldiers, more than 32,000 others wounded and a cost the U.S. more than $700 billion.
Today, the U.S. dropped bombs again on insurgent forces in Iraq as the country is threatened by the forces of ISIS, an al-Qaeda offshoot intent of establishing a caliphate - an Islamic country - in Iraq and neighboring Syria.
Here's a look back at how the situation in Iraq has spiraled out of control since the last American troops departed:
Dec. 15, 2011: U.S. declares end of mission in Iraq
In a small ceremony in Baghdad, the U.S. military formally ended its mission in Iraq after nearly nine years of war. "This is not the end, but the beginning," Defense Secretary Leon Panetta told a small group of U.S. service members and dignitaries gathered to watch the "casing" of the flag of U.S.-Forces-Iraq.
July 23, 2012: Obama touted ending the war in Iraq on bloody day
President Obama's reelection campaign released a video praising the president for ending the war in Iraq on what proved to be the deadliest day of the year in that country. Abu Bakr al-Baghdadi, al-Qaeda's top leader in Iraq, released an audio message announcing, "We are setting off a new stage of our struggle, with the launch of a plan named 'breaking the walls.'"
March 19, 2013: 10th anniversary of Iraq invasion
The 10th anniversary of the start of the U.S. war in Iraq was a muted affair with no official commemorations in either Washington or Baghdad. The anniversary is marred in Iraq by a wave of bombings that killed at least 59 people in Baghdad and injured 221 others.
January 2014: ISIS Forces Take of Fallujah
The predominantly Sunni city in western Iraq was overrun by ISIS. The city had previously been taken by U.S. Marines in one of the bloodiest battles of the Iraq war. Iraq vows to take back the city. Instead, ISIS moves on to take part of the city of Ramadi.
Jan. 17, 2014: U.S. in talks to train Iraqi forces
The Pentagon announced "discussions" with Iraq about the possibility of training Iraq's security forces. Instead, Iraqi security forces receive training in Jordan from other countries in the region.
June 11, 2014: ISIS tightens its grip
In a shocking blitzkrieg, ISIS chased the Iraqi army out of Mosul and swept south to take Tikrit and menace Baghdad. ISIS is aided by Sunni tribes and former members of the Saddam regime.
July 12, 2014 Kurdish Forces Take Control of Kirkuk
Kurdish forces take advantage of the departure of the Iraqi army in the north and take control of the oil-rich city that Kurds had long claimed. The Kurds secure Kirkuk's oil fields, boosting Kurdish hopes to become independent of Iraq.
June 13, 2014 Shiite Cleric Calls for Volunteers
Ayatollah Ali al-Sistani calls for Shiite Muslims to volunteer to fight and protect Shiite shrines. Tens of thousands join militias - not the army - to fight Sunni Iraqis.
June 16, 2014: Marines, U.S. warships head toward Iraq
About 275 Marines and Army troops were deployed to the U.S. Embassy in Baghdad and four warships steamed towards Iraq. ISIS captured the city of Tal Afar.
President Obama sent military advisers to Iraq to assist in training and advising Iraqi forces, but ruled out ground troops returning to Iraq.
June 20, 2014: There are growing calls for Iraqi Prime Minister Nouri al-Maliki, a Shiite accused of marginalizing Sunnis and Kurds, to be replaced in order to be able to unite the country against ISIS. The choice of a new prime minister is still deadlocked.
June 29, 2014: U.S. rushes missiles to Iraq
The U.S. rushed 75 Hellfire missiles to the Iraqi government as it battled to take back the city of Tikrit. The government's effort to recapture Tikrit was repelled by ISIS and its allies.
Aug. 7, 2014: President Obama authorizes airstrikes in Iraq
The president said the U.S. was poised to use air strikes to prevent ISIS from moving on Erbil, the capital of the Kurdish capital where a U.S. consulate is located, or to annihilate stranded members of the Yazidi community who were hiding in mountains.
Aug. 8, 2014: U.S. launches first airstrikes in years
The United States carried out an airstrike against ISIS militants in Iraq, targeting artillery that was firing on Kurdish forces defending Erbil.
ABC's Martha Raddatz, Luis Martinez, Colleen Curry, Hamish MacDonald, Jake Tapper and Alisa Wiersema contributed reporting.