Sunni militant groups stepped up attacks targeting the country's majority Shia population to undermine confidence in the Shia-led government and its efforts to protect people without coalition assistance.

[12] Many Sunni factions stood against the Syrian government, which Shia groups moved to support, and numerous members of both sects also crossed the border to fight in Syria.

However, the war continued for much of the next decade as an insurgency emerged to oppose the occupying forces and the post-invasion Iraqi government.

[19] The United States officially withdrew its troops from Iraq in 2011, but the insurgency and various dimensions of the civil armed conflict continued.

The invasion led to the collapse of the Ba'athist government; Saddam was captured, and he was executed by a military court three years later.

The United States responded with a troop surge in 2007; the heavy American security presence and deals made between the occupying forces and Sunni militias reduced the level of violence.

[21] The Bush administration based its rationale for war principally on the assertion that Iraq possessed weapons of mass destruction (WMDs) and that Saddam's government posed an immediate threat to the United States and its coalition allies.

[22][23] Some U.S. officials accused Saddam of harboring and supporting al-Qaeda,[24] while others cited the desire to end a repressive dictatorship and bring democracy to the people of Iraq.

The Maliki government enacted policies that were widely seen as having the effect of alienating the country's Sunni minority, which worsened sectarian tensions.

As previously planned, the last US combat troops were withdrawn from Iraq in 2011, with security responsibility in the hands of the Iraqi Armed Forces.

[30][31][32][33][34] On 22 December 2011 at least 72 civilians were killed and more than 170 wounded in a series of bombings across Baghdad, while nine others died in various attacks in Baqubah, Mosul and Kirkuk.

On March 5 – A gang of gunmen disguised in military-style uniforms and carrying forged arrest warrants killed 27 police and then hoisted the battle flag of al-Qaeda in a carefully planned early morning attack in Anbar Governorate.

[36] On June 4, A suicide bomber killed 26 people and wounded almost 200 at the offices of a Shiite foundation in Baghdad, sparking fears of sectarian strife at a time of political crisis.

The main cause for upheaval was the ongoing standoff between Vice President Tareq al-Hashemi and Prime Minister al-Maliki, but strained relationships with the Kurdish autonomous regions added to the scene.

On December 23, 2012, several thousand Iraqis marched against al-Maliki, responding to his moves against al-Hashemi and other influential Sunni leaders.

[58][59] In mid-January, a suicide bomber killed a prominent Sunni MP and six others in Fallujah, two days after Finance Minister Rafi al-Issawi survived an assassination attempt in the same city.

The parliamentarian, Ayfan Sadoun al-Essawi, was an important member of the Sons of Iraq committee in Fallujah and part of the opposition to Prime Minister Nouri al-Maliki.

[61][62] Later that month, a suicide bomber blew himself up during a funeral for a politician's relative in the city of Tuz Khormato, killing 42 and leaving 75 others wounded.

[63] In addition, protests by Sunni Muslims in Iraq against the government of Prime Minister Nouri al-Maliki turned deadly in Fallujah, as soldiers opened fire on a crowd of rock-throwing demonstrators, killing 7 and injuring more than 70 others.

Among the wounded was Major General Jamal Tahir, the city's chief of police, who had survived a previous attack at almost the same spot two years earlier.

[68] A week later, ISIS claimed responsibility for the attack, stating that they had "annihilated" a "column of the Safavid army," a reference to the Shia Persian dynasty that ruled Iran from 1501 to 1736.

[69] In mid-March, a series of coordinated attacks across the capital Baghdad and several major cities in the north and central parts of the country killed at least 98 people and left 240 others injured.

Insurgents attacked an oil field near Akaz in a remote part of Al Anbar Governorate, killing 2 engineers and kidnapping a third one.

[72] On April 23, Iraqi Army units moved against an encampment set up by protesters in Hawija, west of the city of Kirkuk, sparking deadly clashes and reprisal attacks across the country.

[73] Insurgents from the Naqshbandi Army completely captured the town of Sulaiman Bek, about 170 km north of Baghdad, after heavy fighting with security forces on April 25, only to relinquish control of it a day later, while escaping with weapons and vehicles.

[75][76][77][78] In late May, the Iraqi government launches Operation al-Shabah (Phantom), with the stated aim of severing contact between Al-Qaeda in Iraq and the Syrian al-Nusra Front by clearing militants from the border area with Syria and Jordan.