War with Iraq is becoming the first major test of the US military services' ability to fight together on the ground and in the air since Vietnam - against an enemy that may know it can't win the war but is showing grim determination not to lose the daily battle.
It's also a test of the military's need in wartime to change approaches - if not plans - in response to a shifting situation on the ground not of its own making and not the ideal for victory.
As planned, the US has opened a "northern front" with the drop of 1,000 paratroopers into areas controlled by Iraqi Kurds. Ideally, that force would have been the Army's 4th Mechanized Infantry Division. Tanks and other heavy equipment from that unit were meant to roll in from the North but, denied access by Turkey, they're steaming around the Arabian peninsula and are probably another week away from the fight.
For this reason and others, the prospect for continued fighting has become weeks if not months.
"We cannot know the duration of the war," President Bush told military personnel and their families at Central Command headquarters in Florida Wednesday in an otherwise cheer-leading speech.
Meanwhile, as the war enters its second week, as one retired senior officer puts it, "desert and distance are taking their toll." Shortages of fuel, ammunition, and spare parts have been reported in a few cases. Due to sporadic fighting, sandstorms, and accidents, some equipment is being degraded and the troops are starting to tire - factors that impact combat readiness.
Before the war started, President Bush indicated that Turkey's refusal to let American ground troops enter Iraq from there would not mean additional risk. But most observers now think that was overly optimistic.
If nothing else, those troops would have been able to challenge Republican Guard units around Saddam Hussein's hometown of Tikrit, halfway down the road south to Baghdad. The regime's leadership, rather than hunkering down in the capital and sending paramilitary and other loyal forces south to harass the approaching Americans, would have to had to fight a war on two fronts instead.
The securing of an important airfield this week by airborne troops may be more important symbolically than militarily at the moment.
They can help secure oil fields and help to separate Turks and Kurds if necessary. But by themselves, they can't project much power. That is likely to come later when more troops and equipment are flown in as expected.
For its part, the Pentagon sees this as part of its plan to keep ratcheting up the war effort in Iraq.
"There isn't an hour or a day that goes by that there aren't an increased number of troops in Iraq in one or more locations," says Defense Secretary Donald Rumsfeld. "And in any given day they go up by a nontrivial number, and they will continue to until it's done."
Senior officers in the war zone hint that light infantry units, such as the Army's 173rd Airborne Brigade which parachuted into northern Iraq this week, could play a key role.
"The capabilities of that force may be more than meets the eye," Army Brigadier General Vincent Brooks said at the Central Command's briefing in Qatar Thursday. "They can be used offensively, but I'm not going to allude to how they may be use as time goes on."
"As we add additional units into the battlefield according to our plan, we increase the number of options we have available," Gen. Brooks said. "And we also increase the number of threats posed to the regime."
Also increasing is the importance of ground and air forces working closely together, particularly as US troops battle Iraqi units close to Baghdad. This involves all four service branches in close coordination in ways not seen for decades.
The last Gulf War started with five weeks of heavy bombing, followed by 100 hours of ground war and then a cease-fire. Combat in the Balkans similarly saw air campaigns and then ground forces to keep the tentative peace.
The war in Afghanistan involved air power and a relatively small number of Special Forces working with anti-Taliban Afghan fighters more used to fighting each other.
That will not be the case in Iraq, where closely-coordinated attacks, including close-air support of ground troops by Air Force, Navy, and Marine Corps tactical jets, heavy bombers, and helicopters, will be key to allied advances.
Some of this has to do with high-tech communications equipment (including real-time video transmitted between units on the ground and in the air) that's never been combat-tested - both to attack the enemy and also to avoid friendly-fire casualties. But it also involves close coordination between services that have historically been rivals.
In terms of organization and training at least, that rivalry has abated in recent years.
But, says military analyst Michael O'Hanlon of the Brookings Institution, "The coming days ... will be the greatest test of our military's commitment to 'jointness' in modern times."