The fall of Saddam's regime created a power vacuum, which, along with the Coalition Provisional Authority's mismanagement, fueled a sectarian civil war between Iraq's Shia majority and Sunni minority, and contributed to a lengthy insurgency.

Retired US Marine, former Navy Secretary and future US senator Jim Webb wrote shortly before the vote, "Those who are pushing for a unilateral war in Iraq know full well that there is no exit strategy if we invade.

[138] In February 2003, the US Army's top general, Eric Shinseki, told the Senate Armed Services Committee that it would take "several hundred thousand soldiers" to secure Iraq.

These efforts consisted of persuading the commanders of several Iraqi military divisions to surrender rather than oppose the invasion, and identifying all the initial leadership targets during very high risk reconnaissance missions.

US forces seized the deserted Ba'ath Party ministries and, according to some reports later disputed by the Marines on the ground, stage-managed[163] the tearing down of a huge iron statue of Saddam, photos and video of which became symbolic of the event, although later controversial.

[164] The abrupt fall of Baghdad was accompanied by a widespread outpouring of gratitude toward the invaders, but also massive civil disorder, including the looting of public and government buildings and drastically increased crime.

[172][173] Muqtada al-Sadr, the leader of a large anti-American faction in Baghdad's Sadr City, issued a fatwa allowing his followers to partake in the looting provided a portion of their takings were gifted to the Sadrist Movement.

[184] To counter this offensive, coalition forces began to use air power and artillery again for the first time since the end of the invasion, by striking suspected ambush sites and mortar launching positions.

The provisional government began training the new Iraqi security forces intended to police the country, and the United States promised over $20 billion in reconstruction money in the form of a credit against Iraq's future oil revenues.

However, violence did increase during the Iraq Spring Fighting of 2004 with foreign fighters from around the Middle East as well as Jama'at al-Tawhid wal-Jihad, an al-Qaeda-linked group led by Abu Musab al-Zarqawi, helping to drive the insurgency.

[170] The most serious fighting of the war so far began on 31 March 2004, when Iraqi insurgents in Fallujah ambushed a Blackwater USA convoy led by four US private military contractors who were providing security for food caterers Eurest Support Services.

[192] Photos of the event were released to news agencies worldwide, causing a great deal of indignation and moral outrage in the United States, and prompting an unsuccessful "pacification" of the city: the First Battle of Fallujah in April 2004.

The Iraq Study Group made up of people from both of the major US parties, was led by co-chairs James Baker, a former Secretary of State (Republican), and Lee H. Hamilton, a former US Representative (Democrat).

The attack may have represented the latest in a feud that erupted earlier that year when members of the Yazidi community stoned to death a teenage girl called Du'a Khalil Aswad accused of dating a Sunni Arab man and converting to Islam.

[231] A statement posted on the Internet by the shadowy Islamic State of Iraq called Abu Risha "one of the dogs of Bush" and described Thursday's killing as a "heroic operation that took over a month to prepare".

144 of the 275 lawmakers signed onto a legislative petition that would require the Iraqi government to seek approval from Parliament before it requests an extension of the UN mandate for foreign forces to be in Iraq, which expires at the end of 2008.

The Bush administration and coalition leaders began to publicly state that Iran was supplying weapons, particularly EFP devices, to Iraqi insurgents and militias although to date have failed to provide any proof for these allegations.

"Government forces have now taken over Islamic militants' headquarters and halted the death squads and 'vice enforcers' who attacked women, Christians, musicians, alcohol sellers and anyone suspected of collaborating with Westerners", according to the report; however, when asked how long it would take for lawlessness to resume if the Iraqi army left, one resident replied, "one day".

"[273] Upon questioning by then Senate committee chair Joe Biden, Ambassador Crocker admitted that Al‑Qaeda in Iraq was less important than the Al Qaeda organization led by Osama bin Laden along the Afghan-Pakistani border.

[296] On 1 January 2009, the United States handed control of the Green Zone and Saddam Hussein's presidential palace to the Iraqi government in a ceremonial move described by the country's prime minister as a restoration of Iraq's sovereignty.

[297] The US military attributed a decline in reported civilian deaths to several factors including the US‑led "troop surge", the growth of US-funded Awakening Councils, and Shiite cleric Muqtada al-Sadr's call for his militia to abide by a cease fire.

US Vice President Joe Biden stated that the deaths of the top two al‑Qaeda figures in Iraq are "potentially devastating" blows to the terror network there and proof that Iraqi security forces are gaining ground.

[372] Defense Secretary Jim Mattis said a tactical shift to surrounding Islamic State strongholds in Mosul, Iraq, and Raqqa, Syria, was devised not only to "annihilate" ISIL fighters hunkered down there, but also to prevent them from returning to their home nations in Europe, Africa, and the Middle East.

[387][388] As of 2021, Iraq had fallen into an economic depression, caused by the ongoing COVID pandemic and falling oil and gas prices, which economists described as the country's biggest financial threat since the rule of Saddam Hussein.

[413] London's International Institute for Strategic Studies concluded in 2004 that the occupation of Iraq had become "a potent global recruitment pretext" for Mujahideen and that the invasion "galvanised" al-Qaeda and "perversely inspired insurgent violence" there.

The report states that the "justification for going to war was based on scanty and deeply flawed intelligence" and that the invasion was an "error compounded by the absence of an agreed exit strategy and the decision to embark on a massive, open-ended nation-building project".

The same report also ascertained that "the occupation authority's first acts were to disband the Iraqi army and the Ba'athist governing party, igniting what would become a lethal, long-running insurgency and eventually a multinational terrorist organization that took over most of the country".

With regard to the Russo-Ukrainian War, Russian foreign minister Sergey Lavrov said in March 2022 that the U.S. exerted similar pressures on Iraq in 2003, which the US invaded later for no reason other than "a vial of unidentified chemicals".

Most significantly, critics have assailed the United States and its allies for not devoting enough troops to the mission, not adequately planning for post-invasion Iraq, and for permitting and perpetrating human rights abuses.

[499][500][501] According to two unnamed US officials, the Pentagon is examining the possibility that the Karbala provincial headquarters raid, in which insurgents managed to infiltrate an American base, kill five US soldiers, wound three, and destroy three humvees before fleeing, was supported by Iranians.