One of the biggest advantages American troops have is psychological. The 1991 war left a vivid impression on the Iraqi survivors. While American airpower was impressive, if not very effective, against Iraqi troops, it was the American ground forces that made the biggest impression, and did the most damage. As has been the case for the last century, the Iraqis found ways to deceive the American smart bombs, and preserve most of their armored vehicles and troops. But when the American tanks and infantry showed up, the Iraqis found they could not deal with it. The most vivid example was the battle of 73 Easting, where a few hundred American troops, outnumbered ten to one, defeated everything the Iraqis could throw at them and led an attack that destroyed the Tawakalna Republican Guard division. Over 100,000 Iraqis came back from the encounter with the American armed forces and the stories they told of magical technology and well trained and disciplined troops has spent over a decade being embellished and expanded in the coffee houses.
The Iraqis might believe they have their own "magical weapons." These would include chemical weapons, urban warfare and hiding behind civilians. The Iraqis have lots of mustard gas, a World War I vintage weapon injures more than it kills, and also sticks around a lot longer than other chemical weapons. During the 1980-88 war with Iran, Iraq used a lot of mustard gas. The Iraqis have also been encouraged by the Palestinian use of street fighting against the Israelis. But the Iraqis are overlooking the fact that the Palestinians lose all those battles. The U.S. has adopted the Israeli tactics, even going as far as buying some of the 50 ton armored bulldozers the Israelis use. Using civilians as shields is a larger problem, but more from a political than a military angle. Civilians that get in the way will get shot at and avoiding this is another chore for the psychological warfare troops.
One problem American ground forces will have advancing on Baghdad is the inability to spread out and advance on a wide front as they did in 1991. That campaign was fought in a vast desert. Once you move north, you enter a densely populated land of rivers and marshes. Coming down from the north, you go through mountainous passes. Under these conditions, the Iraqis can use anti-tank mines and ambushes. These are not wonder weapons, but they do slow the advancing American forces down.
It is unlikely that the 350,000 troops in the regular Iraqi army would offer much resistance. But the Republican Guard and secret police (over 200,000 troops) might resist. One possible reason for such resistance would be to drag out the campaign in the hope that the UN would step in and stop the war. But this is unlikely to work, as it only happened previously when one of the superpowers made that kind of demand. Since the UN does not have the military power of a superpower, because there is only one superpower left, and because that superpower is the one invading Iraq, The other reason for determined fighting is desperation. The many secret police organizations, in particular, are widely hated and liable for revenge attacks once Saddam's government falls. A major, and generally unrecognized, part of the American war effort will psychological warfare. This is a campaign to convince the most hated men in Saddam's government that they can surrender and survive. Exactly how that operation is going to play out is being kept secret, lest Saddam come up with ways to counter the American "psywar" operations.