One of the problems with being a lifelong student of military history is you start assuming everyone knows things that very few people actually do. There have been many times in the past few weeks where I've shouted "DUR!!!" at the TV only to be confronted with a quizzical look from my wife. So, apologies if this all seems a bit too obvious to you, or a gross oversimplification. I'm just trying to make sure we're all working from the same page.
One of the primary, even historic, strengths of the United States military is its reliance on air power. At times we've relied on it too much, and certainly its advocates can be a bit strident. Airpower's popularity took a nosedive after Vietnam, when it became obvious that no amount of bombing could overcome really stupid leadership, but re-thinking strategy and tactics, as well as improved technology and leadership, have brought it back to the fore.
One advantage of airpower is the comparative immunity to interception it provides. In spite of common perception, it's actually extremely difficult to shoot down a modern combat aircraft. What's more, to ensure expensive airplanes and even more expensive pilots are not shot down the first move in any modern battle is to disassemble the other guy's air defenses. Once the sophisticated computer controlled systems are eliminated, aircraft become effectively immune to interception.
Oh, it's still possible to knock them down, especially if the air guys are being stupid (that's what got that stealth fighter shot down over Yugoslavia, not some super-secret development), and there's always just plain dumb luck. Unfortunately you can't rely on stupidity and luck to destroy an entire air force. Once the anti-air system has been brought down, tactically and strategically an air force as a whole becomes unstoppable.
The next advantage is freedom of movement. It's one of those things that is so obvious it's never really examined, but aircraft are fast. It can take hours to move up heavy artillery or big horking battleships within range of a target, while an aircraft can be within range in a matter of minutes. Freedom of movement, and the ability to move rapidly, is a monstrous advantage on a battlefield.
The kind of ordinance (fancy word for "stuff that blows up") an aircraft can put on a target is generally orders of magnitude larger than any other platform available, and this ordinance can be delivered at any point on a battlefield, even far behind the lines. A 2000-pound general-purpose bomb is just a fantastically powerful weapon, quite capable of completely gutting a large office building all by itself. And that's just the biggest general weapon most aircraft can carry. We have a specialized weapon for just about any purpose, and (so far) they all seem to work as advertised.
The traditional weakness of airpower, its lack of precision, seems to have been solved in the past decade. The GPS bomb in particular seems to be a favorite weapon for both close air support and strategic attack. Precision is important not only because it keeps civilian casualties* to a minimum, but because it maximizes efficiency. In WWII, it took 10,000 men in 1000 bombers dropping 8000 bombs to partially disable a single factory. Today that factory can be destroyed by one guy with one bomb.
It's important not to lose sight of the fact that airpower is not an end in and of itself. This was a very hard lesson for the US armed forces to learn, one that has had to periodically be re-learned as time goes by. Airpower, like pretty much every other advanced form of weaponry, is ultimately a support tool for the infantry. The whole point is to make the ground force's job as easy as possible. When used correctly, airpower does this very, very well.
Every ground unit on a battlefield is amazingly vulnerable to attack from above. If guided with precision (and our weapons are nothing if not precise), there is literally no mobile ground unit available today that can withstand even a medium-sized (500-pound) direct hit. They are simply blown to pieces by such things, and again once the air defenses are destroyed there's nothing they can do to defend themselves against such an attack. On a battlefield in which your enemy has achieved air superiority if you move, you die. The problem is you must move in order to fight. Which is why even the fastest armored division is utterly helpless if the other side has free use of the skies.
For a long time the answer was to dig a very very deep hole, crawl in, and pull the door shut behind you. Wait until the invaders got close and then pin them down with whatever you have. Positioned correctly a fortress could be nearly impossible to hit even from the air with unguided munitions, and so the invaders would have to do it the hard way by throwing as many people into your meat grinder as possible in the hopes that someone would get close enough to take you out. If you were smart, you'd have dozens of emplacements like this, making them do it over and over again. Sometimes the other side would stop because there just weren't enough soldiers left alive to continue.
With the development of modern precision guided munitions this doesn't work very well either. Using a slick combination of GPS receivers, radios, and laser designators, a single GI can transmit the exact co-ordinates of your bunker's location to a bomber circling as much as five or six miles overhead. The targeting is so exact that a bomb can literally be put through the gun slot of your bunker, and suddenly your comfy hidey-hole is merely a very short, very noisy ticket to the front of the reincarnation line.
So now you can't move to confront the enemy, and you can't hide in a place he'll have to move past. Even if you set up a temporary ambush you only have a matter of minutes before the sky simply falls on you. And, to repeat, there's nothing, nothing at all, you can do about it.
The only real solution is to take off your uniform and hide in amongst the civilians in the hopes you can start a guerilla campaign against the bad guy's supply lines. If you're lucky, the bad guy is stupid and starts setting villagers on fire in the hopes he'll get the rest of you. In cases like this the villagers will give you food and protection and replacements just to spite the Yankee invader.
If you're unlucky the invaders start setting up their own villages with running water, free cattle, and a plot of land for every peasant able to walk across the lines. If you're really unlucky the guy you work for has made himself extremely unpopular with these villagers already. In such cases the invaders don't even have to do anything special because the traitorous, ignorant, weak, and foolish peasants will turn your ass in faster than you can say "Allah Ahkbar." The last thing you'll notice right before the 500 pound GPS bomb hits will be how there suddenly aren't any peasants around anymore.
What I'm hoping you all take away from this is the understanding that if you hear of an ominous concentration of enemy troops moving toward the front you should be ecstatic, not worried. A large concentration of bad guys on the move is a huge target, not a threat. When you hear about a known concentration of enemy forces dug deep in fortifications you should take heart, because such a concentration is even easier to destroy than guys on the move. The only cause for even a modicum of concern is when the bad guys start trying to melt into the general population. And airpower, combined with the land forces they support, makes it all possible.
What you watch for then is how patient our guys are. The Iraqis have no love for us, because we are invading their country. Never, never lose sight of this fact. It doesn't matter if we're there to "liberate" them... we're big, scary, weird looking, and in their front yard. Only by being patient, by removing the bastards with the sniper rifles and the mortars without killing anyone else will they decide we are the lesser of two evils. When that happens they'll do our work for us, because unlike us they know Achmed's kid Achmad is no damned good and has been playing with some awfully smelly stuff in his basement lately. It is only with the villagers' help that we'll transform a successful suicide bombing into a dumb kid sitting in a cell.