During the war and its aftermath, U.S. forces, joined by allied Sunni groups and later by Iraqi counterterror forces, managed to beat back al Qaeda's Iraqi branch.
But now, Iraqi and U.S. officials say, the insurgent group has more than doubled in numbers from a year ago - from about 1,000 to 2,500 fighters. And it is carrying out an average of 140 attacks each week across Iraq, up from 75 attacks each week earlier this year, according to Pentagon data.
The new growth of al Qaeda in Iraq, also known as the Islamic State of Iraq, is not entirely unexpected. Last November, the top U.S. military official in Iraq, Army Gen. Lloyd Austin, predicted "turbulence" ahead for Iraq's security forces. But he doubted that Iraq would return to the days of widespread fighting between Shiite militias and Sunni insurgents, including al Qaeda, that brought the Islamic country to the brink of civil war.
While there's no sign of Iraq headed back toward sectarian warfare - mostly because Shiite militias are not retaliating to their deadly attacks - al Qaeda's revival is terrifying to ordinary Iraqis.
Each round of bombings and shootings that the terror group unleashes across the country, sometimes killing dozens on a single day, fuels simmering public resentment toward the government, which has been unable to curb the violence. And the rise of Sunni extremists who aim to overthrow a Shiite-linked government in neighboring Syria has brought a new level of anxiety to Iraqis who fear that the same thing could happen in Baghdad.
"Nobody here believes the government's claims that al Qaeda is weak and living its last days in Iraq," said Fuad Ali, 41, a Shiite who works for the government.
"Al Qaeda is much stronger than what the Iraqi officials are imagining. [It] is able to launch big attacks and free its members from Iraqi prisons, and this indicates that al Qaeda is stronger than our security forces."