Although Defense Secretary Donald H. Rumsfeld warned yesterday that the war in Iraq is not over, the U.S. military has already achieved some of its major objectives in just 21 days: a relatively small force has seized nearly two-thirds of a country the size of California, including its capital, while suffering just over 100 dead in combat and accidents.

Military professionals attribute the success to three key elements: a seasoned and well-equipped military, a surprisingly inept Iraqi response, and the decision of the Bush administration at the end of March, when the U.S. and British attack seemed to be faltering in southern Iraq, to keep the Army and Marines focused on Baghdad.

The fighting isn't over yet. Small-scale actions are likely to continue for a week or two, and there still could be major battles in the north. Indeed, administration officials were wary of declaring the war to be finished. "It most assuredly is not over," Rumsfeld said at a Pentagon new conference. "There's going to be some very tough days ahead "

The major cities of northern Iraq still are in the hands of Saddam Hussein's government, and the battle for Tikrit, the Iraqi president's home town about 90 miles north of Baghdad, could be particularly fierce, U.S. defense officials said.

Even so, Rumsfeld did allow himself to acknowledge that "I think it's safe to say that the mood in the country is, in fact, tipping, at least in Baghdad."

Retired military officers and defense experts have discerned two aspects to the U.S. military's role in the war. One was the people and equipment, and the other was the plan they carried out.

"We won so handily because we had a highly professional military -- well-trained, well-outfitted, well-led, with the right doctrine, solid organization, and, most important, excellent people," said retired Army Col. James McDonough.

With some reinforcements arriving in recent days, the U.S. invasion force in Iraq still totals only 125,000 service personnel, a fraction of the half-million troops assembled for the 1991 Persian Gulf War.

In particular, in this war, more than in any previous one, air and ground operations seemed thoroughly meshed, with targeting information between U.S. Special Forces and pilots flowing back and forth as it never has before.

The disadvantage of precision weapons, U.S. officers have noted in previous wars, is that to be used effectively they require precise information. In Iraq for the past three weeks, troops operated in such a way as to find and move that data. Special Operations forces spotted targets and quickly transmitted the information into a system that moved it to bombers orbiting overhead. Reconnaissance drones loitered over the battlefield, showing U.S. commanders enemy movements in real-time video.

When asked yesterday why the U.S. military moves with more agility in combat now than it did during the 1991 Persian Gulf War, a Pentagon official said, "Everything that has a sensor is connected."

The next element was the war plan the commanders followed. "It was an audacious plan, even a risky one," said retired Air Force Lt. Col. Rick Francona, an expert on the Iraqi military. It flung U.S. forces deep into Iraqi territory at the end of an unsecured supply line that suffered repeated ambushes and sniping attacks. It didn't provide for armored reinforcements. And to an unprecedented degree it counted on air power to help out any ground units that got into trouble.

It also was a demanding plan, one that couldn't be executed by slow-moving commanders or poorly trained troops. Pentagon officials cited the plan's "flexibility" so incessantly that it threatened to become a cliche, but the word actually is meaningful in a military sense. Only well-trained forces can quickly change plans, accelerating their movements to take advantage of newly discovered opportunities.

Such last-minute alterations happened several times during the first three weeks of the war, most notably with the ground war being kicked off 24 hours before planned. When the 3rd Infantry Division arrived at the outskirts of Baghdad, rather than ring the city and wait, its commanders decided to pummel Iraqi forces in the capital with armored raids that first established U.S. control over the international airport and, over the last five days, much of the rest of the city.

Similarly, members of the 1st Marine Expeditionary Force threw out their plans yesterday for a slow movement into Baghdad from the east and, with Iraqi authority evaporating, instead drove straight downtown.

The most important aspect of the execution of the plan, said one person who helped develop it, wasn't anything that commanders in the field did, but rather the determination of the Bush administration to stick with it. It was, he said, an approach "that they stuck to even as the U.S. press and some parts of the military, and especially the heavy Army, were doing an outstanding imitation of Chicken Little."

At the end of March, with U.S. and British forces being subjected to guerrilla-style attacks in cities along the invasion route from Kuwait to Baghdad, there was some question inside and outside the U.S. government about whether the Pentagon should turn its attention to consolidating the gains in the south before advancing on the capital. On March 29, President Bush convened a teleconference call from Camp David with his senior national security advisers and decided to keep the military's sights fixed on Baghdad, calculating that the Iraqi capital remained the primary objective in the war.

Randy Scheuneman, president of the Committee for the Liberation of Iraq, an anti-Hussein lobbying group, cited that decision as pivotal. "I think the turning point in the war was the decision to ignore the cacophony of criticism and stick with an aggressive plan based on taking Baghdad rather than liberating Iraq sequentially," he said.

Following that decision, the overarching instruction that Marine Maj. Gen. James N. Mattis, commander of the 1st Marine Division, gave his subordinates was "speed, speed, speed," said one Marine.

That final leap to Baghdad was in some ways the most difficult move of the ground war to date. U.S. commanders already were worried about their strained and vulnerable supply lines. And they expected to be confronted by the Iraqi military's best fighters, the Republican Guard, and perhaps to be attacked with chemical weapons.

"It was a big swallow for the land forces to move out with unsecured flanks and a tenuous logistic supply line," said retired Air Force Gen. Charles Horner, the commander of the air campaign in the 1991 war against Iraq. "But that kept the Iraqis off guard."

That three-day push from southern Iraq to the outskirts of Baghdad during the first three days of April, said Winslow Wheeler, a fellow at the Center for Defense Information, a Washington think tank that is frequently critical of Pentagon policy, "was truly remarkable." He predicted it will be compared by historians to Stonewall Jackson's Virginia campaign in the Civil War and George Patton's march across France in World War II.

There is much that remains unknown about the war. There are indications that the U.S. air campaign was ferociously effective in destroying fielded Iraqi forces, but in contrast to the 1991 war, the air effort has taken a back seat to the ground war this time, both in timing and in the amount of information disclosed about it.

Also, Special Operations teams entered Iraq before the war formally began and roamed Iraq for weeks, calling in airstrikes across the country. "Special Forces have never been used this extensively," said Rep Ike Skelton (Mo.), the senior Democrat on the House Armed Services Committee.

Nor is it known how many Iraqi civilians have died from those airstrikes, most of which have occurred far from television cameras or reporters traveling with military units.

The assessment of military professionals is that U.S. forces in Iraq -- unlike in those two earlier wars -- also have greatly benefited from facing a foe that proved inept.

"Besides the superb performance of U.S. forces, the key to U.S. success was the poor preparedness and lack of organization of the Iraqis," said retired Israeli Army Col. Gal Luft, who commanded Israeli forces in the West Bank towns of Ramallah and Jenin during the 1990s.

The one surprise in the conflict was Saddam's Fedayeen, Baath Party militiamen who blended in with the population and launched ambushes and sniping attacks on U.S. convoys. While those attacks dominated the news early in the war and gave commanders pause, they proved to be more of a nuisance to the military than a genuine impediment. In military terms, said retired Rear Adm. John Sigler, a former chief planner for the U.S. Central Command, "Their impact will be a footnote in the history of this war."

One of the most puzzling aspects of the Iraqi counterattack was their failure to execute any of a dozen strategies that had troubled U.S. officials before the war. Few oil wells were torched, partly because of a preemptive U.S. ground attack in the south. As U.S. forces moved north, the bridges across the Euphrates River weren't dynamited, even though Iraqi combat engineers have demonstrated the ability to do so. Nor were dams blown up to flood the roads.

Most notably, the defenses of Baghdad proved surprisingly easy to attack. Luft was especially critical of Iraqi preparations for urban war. "Abysmal," he said. "No trenches, no barricades, no sniper positions, no booby traps, no mines."

But the first three weeks of the war may eventually be seen as easier than the peace that follows the conflict. Army generals and others interviewed yesterday said they remain extremely concerned about the security in Iraq in the coming days.

"The hard part is yet to come," said retired Army Col. Johnny Brooks, an expert on infantry tactics. "We can easily win the fight but lose the peace."