The guerrillas have widespread popular support in the Sunni Arab areas of Iraq, an area with some 4 million persons. Its cities and deserts offer plenty of cover for an unconventional war. Guerrilla movements can succeed if more than 40 percent of the local population supports them. While the guerrillas are a small proportion of Iraqis, they are very popular in the Sunni Arab areas. If you look at it as a regional war, they probably have 80 percent support in their region.
The guerrillas are mainly Iraqi Sunnis with an intelligence or military background, who know where secret weapons depots are containing some 250,000 tons of missing munitions, and who know how to use military strategy and tactics to good effect. They are well-funded and can easily get further funding from Gulf millionnaires any time they like.
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There are simply too few US troops to fight the guerrillas. There are only about 70,000 US fighting troops in Iraq, they don't have that much person-power superiority over the guerrillas. There are only 10,000 US troops for all of Anbar province, a center of the guerrilla movement with a population of 820,000. A high Iraqi official estimated that there are 40,000 active guerrillas and another 80,000 close supporters of them. The only real explanation for the successes of the guerrillas is that the US military has been consistently underestimating their numbers and abilities. There is no prospect of increasing the number of US troops in Iraq.
The guerillas have enormous advantages, of knowing the local clans and terrain and urban quarters, of knowing Arabic, and of being local Muslims who are sympathetic figures for other Muslims. American audiences often forget that the US troops in Iraq are mostly clueless about what is going on around them, and do not have the knowledge base or skills to conduct effective counter-insurgency. Moreover, as foreign, largely Christian occupiers of an Arab, Muslim, country, they are widely disliked and mistrusted outside Kurdistan.
JuanCole.com