As soon as this evening, President Bush could order the largest US military invasion of another country in 60 years. It will be on familiar ground - the deserts, mountains, and marshlands of Iraq, where a US-led coalition pushed Iraqi troops out of Kuwait 12 years ago.
But this war - which seems inevitable now that Iraqi dictator Saddam Hussein and his sons have rejected a US ultimatum to leave the country - will be very different from the Persian Gulf War of 1991.
The allied coalition is far smaller. And even though American troops are armed with weapons and high-tech equipment far superior to last time, they number only about half the manpower.
On paper, Iraqi forces are outnumbered and vastly outgunned. Some units may refuse to fight, significant numbers of Iraqi soldiers have deserted, and some already have surrendered. But this time they are fighting to defend their homeland from an invading army likened to Christian crusaders - at least that's what President Hussein tells them.
And allied forces have challenges they've never before faced: Finding and destroying chemicals and other weapons of mass destruction; securing and defending two vast oil fields from "scorched earth" tactics; fighting a second front - northern Iraq - over long distances by air and without benefit of a land route from friendly territory (Turkey); securing Baghdad (a city of 6 million people) where Hussein's most loyal troops are clustered; and handling thousands of refugees.
All of this amounts to "a very diffuse set of American objectives," says Larry Seaquist, a retired US Navy warship captain and Pentagon strategist.
"It's alarming to see just how far the military leadership has allowed the Iraq mission package to dilute their forces," says Mr. Seaquist. "I'm hopeful this goes brilliantly with zero casualties, but one wonders at this level of risk."
For one thing, US military objectives would be spread over wide territory.
"This will be about the battle for two important places. Baghdad and the oil-rich city of the north, Kirkuk," says retired Air Force Col. Sam Gardiner, who has taught at the Air War College and the National Defense University. "Baghdad is over 300 miles from the Kuwait border; Kirkuk is another 150 miles beyond that."
The US has not fought at those kinds of distances since World War II, making logistics key for the allies, says Colonel Gardiner.
If this war is full of new challenges for today's US military, it's also likely to be marked by innovations - and perhaps some surprises - in fighting.
"Central Command's master attack plan is full of innovative features not seen in past conflicts," says military analyst Loren Thompson of the Lexington Institute, a public-policy think tank in Arlington, Va.
Among these: From the first day special operations forces will be active destroying Scud missiles, seizing chemical weapons caches, securing oilfields, and capturing Baath Party leaders. A brigade of the 82nd Airborne Division has been charged with accomplishing unconventional missions, probably in Baghdad. Electronic and information warfare will suppress enemy sensors and communications. Ground forces will land deep inside the country rather than advancing along predictable axes in a well-defined front.
In any case, the war is expected to begin with a blitz of airpower followed in short order (within hours or perhaps a few days at most) by soldiers and marines rushing to secure the port city of Basra while other ground units with tanks press north toward Baghdad. Meanwhile, airborne units will hopscotch north as part of "vertical envelopment" - flanking an enemy by going over rather than around him. Special Forces operators - particularly the covert Delta Force - are likely to target Saddam Hussein and other high-ranking holdouts in the regime.
More surprises are likely as well, experts say. A different land route to Baghdad, for example, in which US and allied forces cross the Euphrates River at an unexpected point.
"The most surprising - and most risky - thing the US could do would be a direct strike at a major Baghdad facility by airborne/air assault and Special Forces - all backed by massive air power," says retired Army colonel Dan Smith.
Keep an eye on the 3,000 cruise missiles and precision-guided munitions fired in the first 48 hours, says Colonel Smith, particularly if they're directed at Baghdad airport and Republican Guard divisions. This could be an indicator.
There may be other things to watch for as the fighting wanes.
"I think the surprise will come not in what the United States does but in the consequences," says Gardiner.
"I think we will be surprised at the humanitarian confusion that will come from the combined air/ground operation," he says. "Civilians are more vulnerable to 'shock and awe' than military units. My sense is the civilians will be the first to break."
In public, generals and their field commanders express "can-do" optimism about the capabilities of their forces and eventual victory. But, like Defense Secretary Donald Rumsfeld, they keep long mental lists of what could go wrong. And all officers remember something 19th-century German Field Marshal Helmuth Carl Bernard von Moltke once said, "No battle plan ever survives contact with the enemy."