In May 2003, L. Paul Bremer arrived in Baghdad as the head of the Coalition Provisional Authority (CPA), the civilian administrator of occupied Iraq. His mandate was to oversee the reconstruction of Iraq, the formation of its government, and the transition from military rule to civilian governance.

Two decisions made in that first month would shape the entire war that followed:

First, Bremer issued Order No. 1, which formally dissolved the Iraqi Armed Forces, the Ba'athist Party, and the Ministry of Information. This was called "de-Baathification"—the removal of anyone affiliated with Saddam's party from government and state institutions. Hundreds of thousands of people lost their jobs. The Army, which had over 400,000 personnel, was gone.

Second, Bremer issued Order No. 2, dissolving all official and private militias not integral to and operating under the control of the Ministry of Interior or Defense. This was meant to disarm sectarian groups, but it was unevenly enforced.

These orders had been discussed with Pentagon officials and had support from elements of the Bush administration, particularly those who believed that a complete break with the past was necessary. But they had catastrophic consequences.

The first consequence was economic. Hundreds of thousands of armed, trained men suddenly had no jobs and no income. Many had families, some had combat experience, and all had grievances. They constituted a pool of potential insurgents.

The second consequence was political. The de-Baathification order didn't just remove high-level officials; it removed teachers, engineers, government officials, police, and military officers who had been party members not necessarily out of ideological commitment but out of professional necessity. In the Iraqi system, party membership was a prerequisite for many government jobs. Removing all of them from government created a vacuum in state capacity and alienated a large portion of the Sunni Arab population, which had dominated the state under Saddam.

The third consequence was security. The disbanding of the Army removed one of the few institutions with the training and capability to provide security. The new Iraqi military would have to be built from scratch, but that would take time.

By the fall of 2003, insurgent groups were operating openly in Sunni-majority provinces. Some were Ba'athists or members of Saddam's military apparatus. Others were Islamic extremists, some of them veterans of the jihadist wars in Afghanistan and Chechnya. Still others were Shiite militias operating against the occupation or against Sunni rivals. The conflict was becoming sectarian.

The most dramatic moment in this period came in April 2004 in Fallujah. The city, about 40 miles west of Baghdad, had become an insurgent stronghold. In March 2004, insurgents attacked a convoy carrying private security contractors, killing four. The bodies were desecrated and hung from a bridge over the Euphrates River.

The American response was an assault on the city. For two weeks in April, American Marines and Army units fought house-to-house in Fallujah. The fighting was fierce, casualties mounted, and the civilian cost was significant. The pictures of dead children and destroyed homes became iconic images of the war.

Fallujah April 2004 revealed something profound about the conflict. This was not terrorism that could be defeated by air strikes and special operations raids. This was urban insurgency, in cities where the U.S. military had overwhelming firepower but no way to use it without killing civilians. It was the most difficult type of warfare: counterinsurgency in populated areas against enemies who chose when and where to fight.

The same period also saw the rise of the Mahdi Army, the militia of Moqtada al-Sadr, a Shiite cleric. In August 2004, the Mahdi Army launched an insurgency against the occupation, fighting both American forces and the emerging Iraqi security forces aligned with the more moderate Shiite establishment.

By the end of 2004, American forces were fighting on multiple fronts: against Sunni insurgent groups, against jihadists, against Shiite militias. The conflict was becoming more complex and more integrated with Iraqi sectarian politics. The insurgency was no longer a reaction to the occupation; it was becoming an expression of the fundamental conflicts within Iraqi society: between Sunni and Shia, between Arab and Kurd, between different visions of what post-Saddam Iraq should be.

The casualties mounted. Americans killed in action: 486 in 2003, 848 in 2004. Iraqi military and police: hundreds. Civilian deaths: thousands. And the insurgency was growing stronger, not weaker. The quick victory was becoming a long war.