The first cause of the Spring Fighting was the rise of a conservative Shiite cleric Muqtada al-Sadr and his militia, the Mahdi Army, in the south of the country.

With the fall of Saddam Hussein, Muqtada al-Sadr emerged as a Shia leader by rejecting the US-led occupation of Iraq.

The four independent contractors were guarding food shipments for a U.S. base on the outskirts of Fallujah, Iraq, when they took a wrong turn and entered the city.

[7][8] On March 28, the U.S. overseer of Iraq, Paul Bremer, ordered the 60-day closure of Al-Hawza, a newspaper published by Muqtada al-Sadr's group, on the charges of inciting violence against the occupation.

[10] On April 3, Bremer sent troops to al-Sadr's home and arrested Mustafa Yaqoubi, a top lieutenant,[10] sparking further protests.

On April 4, Spanish-led troops clashed with armed demonstrators in Najaf demanding Yaqubi's release, resulting in the death of two coalition soldiers, one American and one Salvadoran, and at least 20 Iraqis.

[11] The main coalition base at Najaf was defended by Salvadoran troops, Blackwater contractists and Spanish armoured vehicles, which repelled the attack of several technicals.

[12] The same day al-Sadr issued a statement calling on his supporters to stop staging demonstrations "because your enemy prefers terrorism.

Unrest in Basra was also inflamed by the publication of fake torture pictures by Piers Morgan whilst editor of the British Newspaper The Mirror with one officer briefing as such: "Col Black told the press conference: "These photographs are a recruiting poster for al-Qa'eda and every other terrorist organisation.

In what was described as a 5-mile long ambush, the 26-vehicle column was pummeled by gunfire, mortar rounds and RPGs, disabling many of the civilian fuel tankers and Army vehicles.

At the beginning of May, coalition forces estimated that there were 200-500 militants still present in Karbala, 300–400 in Diwaniyah, an unknown number still left in Amarah and Basra, and 1,000-2,000 still holed up in the Najaf-Kufa region.

In response to the killing of the four Americans on March 31 and intense political pressure, the U.S. Marines commenced Operation Vigilant Resolve.

[20] They surrounded the city with the intent of capturing the individuals responsible for the killings, as well as others in the region who might have been involved in the insurgency or terrorist activities.

It was planned that the Iraqi National Guard would fight alongside the U.S. Marines in the operation, but on the dawn of the invasion they discarded their uniforms and deserted.

Heavy fighting lasted until April 9, 2004, when, again under enormous public pressure, the offensive was called off because of great civilian losses.

On April 10, the U.S. military declared a unilateral truce to allow for humanitarian supplies to enter Fallujah, and pulled troops back to the outskirts of the city.

Local sheikhs and imams refused to honor the cease-fire agreement, and repeatedly sent mujahideen fighters to attack the Marines.

Military commanders said their goal in the siege was to capture those responsible for the numerous deaths of American and Iraqi security personnel.

It was also reported that the Marines wanted a cease-fire because they were not being resupplied, due to the insurgent capture of the main highway from Baghdad to Anbar.

Soon afterwards, many towns in Anbar province - such as Karabilah, Sada, Romania, Ubaydi, Haqlaniyah, Hit, Baghdadi, Haditha, as well as numerous smaller villages - came under insurgent control.

Several individuals, including an American civilian (Nicholas Berg) and a South Korean (Kim Sun-il), among others, were beheaded during this period.

At the same time, perhaps as a diversionary tactic, hundreds of Mahdi Army insurgents swept through Basra, firing on British patrols and seizing parts of the city.

Twenty-one members of the Iraqi security forces, two American soldiers and thirteen civilians were killed during the street fighting in Baquba.

Also in November the Second Battle of Fallujah occurred, when US forces carried out Operation Phantom Fury, which left ninety-five percent of the city in ruins.

Four days after the end of the Spring Fighting on June 28, 2004, the Coalition Provisional Authority transferred control to a new Iraqi government.