Lib, it's more their country then yours. Maybe 10% of the insurgency at most are foreign fighters, though these tend to be among the higer echelons due to their previous expertise (Afghanistan in the 80s, Bosnia etc). US brass in Iraq has confirmed this. Most of the rank and file is pissed-off Iraqis, and until America faces that fact, victory isn't even on the horizon.
What you are suggesting is more or less what the Russians did to Chechnya. They park their artillery outside Grozny, and turn the place to rubble. And send in some bombers for good measure. Unfortunately, that resulted in about 100,000+ civilian deaths, and the destruction of an entire country. In technical terms, genocide, ethnic cleansing (there are only around 1.3 million Chehcens left) and war crimes. Didn't work though, the guerillas just went to the hills and fought from there. The point is, people resent others meddling in their country. Regardless of whether you think it's good or bad, right or wrong, the very presence of foreign troops, and their control over the civilian government, that by itself creates huge resentment.
Yes, dictators exist. Yes, it's a tricky situation to unseat them. But firstly, violence is never the solution to internal conflicts. Wars between nations, yes, but civil war and insurgency/counter-insurgency are incredibly messy, to put it mildly, and rarely achieve anything. People resent, quite naturally, when some outside power comes along and tries to fix all the problems, like some sort of global cop. Even in a completely positive situations, internal problems are best handled very, very gently. The main thrust has to come from within, or else people will not feel like they have a stake in their own affairs. The civil wars in Africa can be solved, though it takes efforts on many fronts. The African Union could potentialkly be a force for great good, if they weren't so ineffectual. If America likes to solve thing forecefully, fine: African Union peacekeepers are extremely ill equipped and trained. Handing out weapons and training is the US's specialty, I don't see why they couldn't do that in Africa too.