WASHINGTON — Two days after Hurricane Katrina hit the Gulf Coast, President Bush went on national television to announce a massive federal rescue and relief effort.

But orders to move didn't reach key active military units for three more days.

Once they received them, it took eight hours for 3,600 troops from the 82nd Airborne Division at Fort Bragg, N.C., to be on the ground in Louisiana and Mississippi with vital search-and-rescue helicopters. An additional 2,500 soon followed from the 1st Cavalry Division at Fort Hood, Texas.

"If the 1st Cav and 82nd Airborne had gotten there on time, I think we would have saved some lives," said Gen. Julius Becton Jr., director of the Federal Emergency Management Agency (FEMA) under President Reagan from 1985 to 1989. "We recognized we had to get people out, and they had helicopters to do that."

Federal officials long have known that the active-duty military is the only organization with the massive resources and effective command structure to handle a major catastrophe.

In a 1996 Pentagon report, the Department of Defense (DOD) acknowledged its large role in major disasters. Between 1992 and 1996, the Pentagon provided support in 18 disasters and developed five training manuals on how to work with FEMA and civilians in natural disasters.

"In catastrophic disasters, DOD will likely provide Hurricane Andrew-levels of support and predominately operate in urban or suburban terrain," the report said. "This should be incorporated into planning assumptions."

The delay this time in tapping the troops, helicopters, trucks, generators, communications and other resources of the 1st Cavalry and the 82nd Airborne is the latest example of how the federal response to Katrina lacked organization and leadership. And it raises further questions about the government's ability to mobilize the active-duty military rapidly now that FEMA has been absorbed into the massive, terrorism-focused Department of Homeland Security.

Addressing the nation Thursday night from New Orleans, Bush said the storm overwhelmed the disaster-relief system. "It is now clear that a challenge on this scale requires greater federal authority and a broader role for the armed forces, the institution of our government most capable of massive logistical operations on a moment's notice," he said.

Several emergency-response experts, however, questioned whether Bush and Homeland Security Secretary Michael Chertoff understood how much authority they had to tap all the resources of the federal government — including those of the Defense Department.

"To say I've suddenly discovered the military needs to be involved is like saying wheels should be round instead of square," said Michael Greenberger, a law professor and the director of the University of Maryland's Center for Health and Homeland Security.

During the last great hurricane — Andrew in 1992 — the failure to send food, water and shelter to Florida and to victims highlighted the importance of quickly engaging the Pentagon.

"For such disasters, DOD is the only organization capable of providing, transporting and distributing sufficient quantities of items needed," the Government Accountability Office, the investigative arm of Congress, wrote in a 1993 report.

Indeed, the new National Response Plan, the nation's blueprint for responding to disasters that was unveiled with much fanfare in January by Chertoff's predecessor, Tom Ridge, includes a section on responding to catastrophic events.

"Unless it can be credibly established that a mobilizing federal resource ... is not needed at the catastrophic incident venue, that resource deploys," the plan says. The plan and a 2003 presidential directive put Chertoff, as Homeland Security secretary, in charge of coordinating the federal response.

Chertoff, who aides said has been engaged in the response to Hurricane Katrina, went to Atlanta the day after the storm hit for a previously scheduled briefing on avian flu.

Aides also concede that Washington officials were unable to confirm that the levees in New Orleans had failed until midday Tuesday, Aug. 30. The breaches first were discovered in Louisiana some 32 hours earlier.

Greenberger, the Maryland homeland-security expert, said he wonders whether Chertoff and other top federal officials understand the National Response Plan or even had read it before Katrina.

"Everything he did and everything he has said strongly suggests that that plan was never read," Greenberger said of Chertoff.

Chertoff was in Gulfport, Miss., yesterday to participate in the Harrison County National Day of Prayer and Remembrance. He took no questions from reporters. Homeland Security officials didn't return calls for comment.