Guerrilla warfare is a modern way of waging war against an army that is superior to your own. It dates back to the Napoleonic Wars, and was (perhaps) invented by the Spanish landowners and peasants who fought off the French Army. Remember that Napoleon lost a two front war: while his troops were fighting in Russia, they were also fighting in Spain. This is remembered today as the Spanish War of Independence, although Spain was only briefly 'dependent' on France, and it is where we get the word 'guerrilla' or 'little warrior'.
Guerrilla warfare was revived in the Twentieth Century by Mao's troops in China. Here, as well, these were peasant fighters acting against an urban army. Guerrilla troops have been compared to a slowly-tightening noose that comes from the rural area and closes in upon the urban.
Guerrilla warfare is worth understanding because it is largely the method that has been used against US troops in Afghanistan and Iraq. While the US military is superior to any in the world in regards to traditional warfare (which is why they defeated the Iraqi military in a matter of days), they are generally not good at handling guerrillas. Neither are most traditional Western armies. The Spanish guerrillas defeated Napoleon, remember? The French were later mired and defeated by guerrilla fighters in Algeria, and the US by the same in Vietnam.
Guerrilla warfare is ideal for an army that cannot possibly win in face-to-face combat on a level field of battle. It is carried out in secret, often in the dead of night. It involves methods of ambush, booby traps, and snipers. The goal of guerrilla warfare is to force the superior army to expend their energy and resources. What makes guerrillas particuarly difficult for armies to deal with is that they blend into the surroundings so easily. Usually, but not always, indigeneous to the area, they have superior communications networks, and can hide themselves among the local population.
One reason that Western armies may fall prey to guerrillas so easily is that they see them as lowlifes and criminals, and expect civilians to think the same. This is not always the case.
For the guerrillas, the trick is to force the invading army to waste its time and resources trying to capture the 'banditos'. Guerrilla bands tend to be fairly decentralized, by necessity, which can make them more difficult to break. However, this can also work against the guerrillas; rival groups of guerrillas can start working against each other. Also, there will sometimes arise groups of bandits, essentially, who will take adavantage of the chaotic situation to better themselves. Some of these groups are simply opposed to any sort of social order being reimposed. They can force a breakdown in ethical standards throughout the society. Therefore, in guerrilla wars, violence can reach astounding levels of savagery.
Guerrillas tend to terrorize the local population to some extent- generally through targeted executions of 'collaborators' or those who wish to remain neutral. In reagrds to Iraq, we can see a general effort to make life unlivable for civilians. However, this can easily backfire, and I think it has in Iraq. Guerrillas have to terrorize civilians, but they also have to provide protection for loyal civilians, in much the same way as any organized crime network does. In Iraq, we see the danger that guerrillas face in 'overdoing' the use of force. The population, to the best of my knowledge, is still opposed to the 'insurgents' who seem to be indiscriminate in their use of force. Also, these guerrillas are often newcomers to the region. Local guerrillas can provoke nationalistic feelings (resisting the occupiers for the homeland), while the insurgents cannot do so in Iraq.
For the occupying army, the trick is to best protect the civilian population, including through the use of police repression, without turning the society into a prison camp. This is where the French failed in Spain and Algeria, and where the US has had the most trouble in Iraq. It is necessary to rid the population of guerrillas through military sweeps, or what the French called a ratissage (raking over) in Algeria. However, if force becomes excessive, or carries on for too many years, the population will turn against the occupying army. Understandably, they want protection; in a Hobbsian sense they want social order. But, they do not want to be unduly repressed by their protectors. One main reason to oppose the use of torture is exactly what we have seen in Iraq- civilians who still rely on the US presence for protection, but who have been horrified by images of torture. Torture can never be kept under wraps- not without 'disappearing' its victims. Moreover, it displays a lack of control, and a disloyalty to the civilian population. It acts directly against the interests of the army that uses it.
What the military that is fighting the guerrillas needs to do is maintain order, while still allowing for a relative amount of liberty on the part of the civilian population. They need to be allies of the civilians, instead of superior-minded invaders. What this means, especially in Iraq, is being aware of the local culture. In Spain, Napoleon's troops turned the conservative elements of society against their army by repressing the local Priests. The Revolutionary French were much more anti-clerical than the Spanish were. In Iraq, it is most important to work in concert with more conservative religious insititutions. For instance, protecting the mosques should be a top priority. And troops should be forbidden from using anti-Islamic interrogation techniques, which is as stupid as torture given our military goals.
Moreover, they should work to publicize the attrocities committed by the guerrillas, while also publicizing whatever areas they themselves have secured. Even something as benign as paving a road is a vital step towards restoring order, and should be treated as equally important to combat missions. In a sense, the scenes of troops giving toys to children helps. But, they also need to supply basic services to those childrens' parents. To this end, the US needs more troops than they have now, and may even consider the use of a draft.
Since this is a sectarian conflict in Iraq, the US should use groups of insurgents against each other. This may involve making deals with specific groups and using those groups to repress their rivals. Obviously, this will be ugly business, very ugly business, in fact. However, I don't think the US can put down rival groups of guerrillas while trying to stop a potential civil war from developing. At this point, the best hope they might have is in finding the least noxious group of guerrillas and deputizing them.
The situation in Iraq is quite-likely doomed for the US. However, I don't believe that an 'immoral' war can be made more moral by US troops leaving the region to fall into chaos. I think the US needs to take responsibility for the mess it's made, and try to leave a stable and functioning state. By any account, the situation is FUBAR. But, that is the cross that the US should bear. Here are my suggestions to fight the guerrillas in Iraq:
1) Secure all public services and keep them manned by troops for the indefinite future. Things like water and electricity must be kept functioning.
2) Try to get the support of every willing nation, including Iran, in securing the nation.
3) Secure the borders first, and then work inwards, tightening the noose.
4) Police sweeps are, unfortunately, necessary. Torture, however, is a quick way to convince civilians that we are the enemy.
5) All works that are usually done by the peace corps: such as road-building, bridge-repair, well-digging, and so forth, are critical here. Life must return to normal as much as is humanly possible.
6) For God's sakes, learn the damn culture! And be aware not to present yourselves as being against that culture. As occupiers, the best we can be seen as is protectors. The worst thing we can be seen as is the enemy of the average Iraqi.
7) Demonize the insurgents in the popular media- Propaganda is critical here.
8) Use insurgent groups against each other.
9) Accept the fact that a functioning Iraq state may well be run by Sunni Muslims and sharia law. This is preferable to Shi'ite rule in that Sunnis distinguish between the politcal ruler and the religious rulers, while shi'ite do not. A secular democracy in Iraq is not going to happen.