The Iraq spring fighting of 2004 was a major series of offensives undertaken by Sunni and Shia Iraqi insurgents against the American and British-led MNF-I coalition forces in Iraq and their New Iraqi Army allies in April-June 2004.
After the fall of Saddam Hussein, the Shia leader Muqtada al-Sadr rejected the US-led occupation of Iraq and created the Mahdi Army in June 2003. Concurrently, Sunni resistance intensified in the city of Fallujah, resulting in the killing of four Blackwater contractors on 31 March 2004. After the US overseer of Iraq Paul Bremer ordered the 60-day closure of a radical Sadrist newspaper on 28 March 2004, thousands of Sadrists protested, and further protests were sparked when Bremer had Sadr's lieutenant Mustafa Yaqoubi on 3 April. On 4 April, an American soldier and a Salvadoran soldier were killed in clashes with armed Sadrist demonstrators in Najaf. A day after Sadr called on his supporters to stop staging demonstrations, violent protests occurred throughout the Shi'ite south, turning into a rebellion by 6 April 2004. The Mahdi Army began an offensive in Najaf, Kufa, Kut, and Sadr City, taking control of public buildings and police stations. Soon, the fighting spread to other parts of Baghdad, and the militants gained partial control of Karbala. The heavily embattled Spaniards were withdrawn after a new government took power in Spain. After sporadic clashes, Coalition forces suppressed militia activity in Nasiriyah, Amarah, and Basra before retaking Kut on 16 April. 2,500 Coalition troops besieged Najaf, where Sadr was suspected to be hiding, but the Coalition loosened their stranglehold in order to negotiate with the Mahdi Army.
On the same day as the Shia uprising, the US Marine Corps launched Operation Vigilant Resolve in Fallujah to capture the Sunni militants responsible for the Blackwater killings. The Iraqi National Guardsmen slated to participate in the offensive deserted, and the offensive was called off on 9 April due to heavy civilian losses; by then, the Marines had only taken 25% of the city. At the same time, heavy fighting raged in Ramadi. On 9 April, the Americans allowed 70,000 women, children, and elderly residents of Fallujah to leave the city and declared a unilateral truce on 10 April, but local sheikhs and imams refused to honor the ceasefire and ordered attacks on the Marines. On 1 May 2004, US forces completely withdrew from the city, and the brigade they sent to garrison the city went over to the insurgents due to its Ba'athist sympathies. Soon, the Anbar Governorate towns of Karabilah, Sada, Romania, Ubaydi, Haqlaniyah, Hit, Baghdadi, and Haditha went over to the insurgents as well.
In May 2004, the US launched a counteroffensive against the Mahdi Army in southern Iraq, retaking Karbala on 24 May. Najaf and Kufa experienced sustained American assaults, with Sadr continuing to give Friday sermons in Kufa during the battle. On 30 May, insurgents took over Samarra after American forces withdrew from its interior, and they also cut off the Baghdad-Karbala highway. On 6 June 2004, Sadr ordered his army to cease operations in Najaf and Kufa, but the fighting continued until 24 June 2004. On that day, Sunni insurgents went on the offensive in Ramadi, Baghdad, Mahmudiya, Baqubah, and Mosul. The most intense fighting occurred in Mosul and Baqubah, and insurgents were able to capture Ramadi. By the end of the insurgent offensive, Anbar Governorate fell into the hands of the insurgents, while the USA retained control of Baghdad and other major cities in the Shia south. However, a new Shia uprising resulted in the Battle of Najaf, while a Second Battle of Fallujah in November 2004 left 95% of the city in ruins.