The broad outlines of the decision are now widely known, defended by proponents as necessary to ensure that Saddam Hussein’s influence did not outlive his ouster from power.

But with the fifth anniversary of the start of the war approaching, some participants have provided in interviews their first detailed, on-the-record accounts of a decision that is widely seen as one of the most momentous and contentious of the war, assailed by critics as all but ensuring that American forces would face a growing insurgency led by embittered Sunnis who led much of the army.

The account that emerges from those interviews, and from access to previously unpublished documents, makes clear that Mr. Bremer’s decree reversed an earlier plan — one that would have relied on the Iraqi military to help secure and rebuild the country, and had been approved at a White House meeting that Mr. Bush convened just 10 weeks earlier.

The interviews show that while Mr. Bush endorsed Mr. Bremer’s plan in the May 22 meeting, the decision was made without thorough consultations within government, and without the counsel of the secretary of state or the senior American commander in Iraq, said the commander, Lt. Gen. David D. McKiernan. The decree by Mr. Bremer, who is known as Jerry, prompted bitter infighting within the government and the military, with recriminations continuing to this day.