The initial outbreak of violence (the 2003–2006 phase) was triggered by the fall and preceded the establishment of the new Iraqi government by the Multi-National Force – Iraq (MNF–I), which was led by the United States.

By February 2006, the violence escalated into a Shia–Sunni civil war, and for the next two years, the MNF–I and the Iraqi government were locked in intense fighting with various militants, who were also targeting each other based on their sectarian affiliations.

While the special forces launched an amphibious assault from the Persian Gulf to secure Basra and the surrounding petroleum fields, the main invasion army moved into southern Iraq, occupying the region and engaging in the Battle of Nasiriyah on 23 March.

Following the U.S.-launched 2003 invasion of Iraq, the situation deteriorated, and by 2007, the intercommunal violence between Iraqi Sunni and Shi'a factions was described by the National Intelligence Estimate as having elements of a civil war.

[24][25] The Red Cross has also stated that Iraq's humanitarian situation remains among the most critical in the world, with millions of Iraqis forced to rely on insufficient and poor-quality water sources.

[31] Steven Simon, a Senior Fellow for Middle Eastern Studies at the Council on Foreign Relations, wrote in May 2008 that "the recent short-term gains" had "come at the expense of the long-term goal of a stable, unitary Iraq.

"[33] In 2010, the low point for the al-Qaeda effort in Iraq, car bombings declined to an average of ten a month and multiple-location attacks occurred only two or three times a year.

By motivation, essentially, and with the exception of the Al Qaeda in Iraq element who have endeavoured to exploit the situation for their own ends, our opponents are Iraqi Nationalists, and are most concerned with their own needs – jobs, money, security – and the majority are not bad people.

The pre-war organization of the Ba'ath Party and its militias as a cellular[citation needed] structure aided the continued pro-Saddam resistance after the fall of Baghdad, and Iraqi intelligence operatives may have developed a plan for guerrilla war following the toppling of Saddam Hussein from power.

As the goal of restoring the Ba'ath Party to power was seemingly out of reach, the alternative solution appeared to be to join forces with organisations who opposed the U.S.-led invasion.

On 30 November 2003, a U.S. convoy traveling through the town of Samarra in the Sunni Triangle was ambushed by over 100 Iraqi guerillas, reportedly wearing trademark Fedayeen Saddam uniforms.

Ad-Douri had taken over the running of the Iraqi Ba'ath Party following Saddam Hussein's capture in 2003 and had been endorsed by a previously unknown group calling itself Baghdad Citizens Gathering.

[41][42] One former minister in the interim government, Ayham al-Samarai, announced the launch in 2005 of "a new political movement, saying he aimed to give a voice to figures from the legitimate Iraqi resistance.

Colonel Derek Harvey told Reuters "that the U.S. military detained Badr assassination teams possessing target lists of Sunni officers and pilots in 2003 and 2004 but did not hold them.

"There seems to have been a strategic decision taken sometime over late winter or early spring by Damascus, Tehran, along with their partners in ait Lebanese Hezbollah, to provide more support to Sadr to increase pressure on the U.S.," the American intelligence official said.

One particular document, which was apparently written after he lost power, appeared to be a directive to his Ba'athist loyalists warning them to be wary of Islamist mujahideen and other foreign Arabs entering the country to join the insurgency.

A U.S. official commenting on the document stressed that while Saddam urged his followers to be cautious in their dealings with other Arab fighters, he did not order them to avoid contact or rule out co-operation.

Bruce Hoffman, a Washington counter-terrorism expert stated that the existence of the document underscores the fact that "this is an insurgency cut of many different cloths...[and] everybody's jockeying for their position of power in the future Iraq."

[53] Foreign fighters are mostly Arabs from neighboring countries, who have entered Iraq, primarily through the porous desert borders of Syria and Saudi Arabia, to assist the Iraqi insurgency.

It is generally believed that most are freelance fighters, but a few members of Al-Qaeda and the related group Ansar al-Islam are suspected of infiltrating into the Sunni areas of Iraq through the mountainous northeastern border with Iran.

Zarqawi was considered the head of an insurgent group called Al-Tawhid Wal-Jihad ("Monotheism and Holy War") until his death on 7 June 2006, which according to U.S. estimates numbers in the low hundreds.

The paper quoted University of Michigan history professor Juan Cole: "If the Iraqi Sunni nationalists could take over their own territory, they would not put up with the few hundred foreign volunteers blowing things up, and would send them away or slit their throats.

[citation needed] U.S. Army Specialist Tony Lagouranis spoke about his job identifying many of the bodies after the assault on Fallujah: We had women and children, old men, young boys.

[68]In July 2007, the Los Angeles Times reported that 45% of all foreign militants targeting U.S. troops and Iraqi civilians and security forces are from Saudi Arabia; 15% are from Syria and Lebanon; and 10% are from North Africa.

"[75] Large-scale terrorist attacks against civilians carried out by foreign fighters, as well as the interpretation of Islam that they attempt to impose on the local population in areas under their control, have increasingly turned Iraqis against them, in some cases breaking out into open fighting between different groups in the insurgency.

Terrorism expert Jessica Stern warned that; "In the run-up to the war, most Iraqis viewed the foreign volunteers who were rushing in to fight against America as troublemakers, and Saddam Hussein's forces reportedly killed many of them.

[85] In response to al-Qaeda killings in Iraq, Sunni insurgents in al-Anbar province led by former Ba'athist intelligence officer Ahmed Ftaikhan formed an anti-al-Qaeda militia called the Anbar Revolutionaries.

"[86] The schism became all the more apparent in when a tape alleged to be from the Mujahedeen Shura Council urged Osama Bin Laden to replace al-Qaeda in Iraq's current head with an Iraqi national.

The insurgents are believed to maintain a key supply line stretching from Syria through al-Qaim and along the Euphrates to Baghdad and central Iraq, the Iraqi equivalent of the Ho Chi Minh trail.

Iraqi security and police forces had also been significantly built up in the capital and, despite being constantly targeted, had enjoyed some successes such as the pacification of Haifa Street, which however subsequently saw a massive surge of insurgent activity.