The battle of Fallujah, in late 2004, is still being studied by U.S. Army and Marine historians and doctrine ("how to fight") experts. The Fallujah fighting was quite intense, even by historical standards, something that the media missed. What was noticed was how quickly the army and marine troops blitzed through the city, clearing out the 4,000 very determined defenders. The speed and efficiency of the American attack was the result of some unique, in the history of warfare, factors. But the principal reason for the success in Fallujah was the high degree of training the troops had. Many also had months of combat experience in Iraq. These factors (training and combat experience) have long been key factors in combat success. But the American troops in Fallujah had some relatively new advantages, that were used aggressively. These included massive amounts of information on the enemy, and robotic weapons. The standard gear of the 5,000 attacking troops was also exceptionally good by historical standards. Especially notable was the improved body armor and communications gear.
The end result of all this was a two week campaign that resulted in some 500 American and Iraqi casualties, but the obliteration of the defending force (1,200, 1,500 captured, the rest either got out, or were buried in bombed buildings). While the enemy were not, compared to the U.S. troops, well trained, they were motivated, and often refused to surrender. But the speed and violence of the American assault prevented any coordinated defense. The U.S. troops quickly cut the city into sectors, that were then methodically cleared out.
The terrorists that got out, later all repeated the same story. Once the Americans were on to you, if was like being stalked by a machine. The often petrified defender could only remember the footsteps of the approaching American troops inside a building, the gunfire and grenade blasts as rooms were cleared, and the shouted commands that accompanied it. If a building was so well defended that the American infantry could not get in, they would just obliterate it with a smart bomb. They used smaller weapons, like AT-4 rocket launchers, many of which fuel-air explosive (thermobaric) warheads. These would use an explosive mist to create a lethal blast, capable of clearing several rooms at once. The defenders could occasionally kill or wound the advancing Americans, but could not stop them. Nothing the defenders did worked, and the American tactics developers want to keep it that way.
The speed with which intelligence information (from troops, electronic intercepts, and constant live video via UAVs and gunships overhead) was processed enabled commanders to keep the battle going 24/7. The defenders were not ready to deal with this, and many of them died while groggy from lack of sleep. When in that condition, you are more prone to make mistakes, and the attackers were ready to take advantage.
Compared to earlier wars, there has never been anything quite like Fallujah. The Pentagon is still sorting out what it all means for the future of warfare. What they do know is that future battles are likely to continue being different that anything in the past.