Al-Qaeda's (and other terrorists') attempts to kill Muslims (primarily) in Iraq and elsewhere have hardly much to do with the presence of US soldiers trying to stop them. If American soldiers would not stand between Sunni and Shia extremists, how likely would it be that they would fight again and cause _millions_ of deaths like in the 80s and 90s? You are creating the impression as if you believed that Al-Qaeda blow up Shiite places of worship because of America. They don't. They do it because they hate Shiites (and Jews and Persians and Sunnis who don't subscribe to their heretical Islam). Shia militias are also well-known for attacking Sunnis/Christians/Jews/whomever.
The current situation in Iraq can be attributed not to the U.S invasion of Iraq, but the in the days following after it. Despite the fact that the invasion was illegal and it's reasons unfounded (catguy is correct that Iraq had no WMD's or no connection to 9/11 etc) the overall sentiment of the general Iraqi population was very high shortly after the U.S took over. People were actually optimistic and hopeful to see what life would be like without Saddam. In all honesty, even though I completely disagree with the invasion of Iraq, if the Bush administration had played it's cards differently they actually could have come out as heroes for both U.S and Iraqi's alike. Let's put aside the rhetoric, jingoism and ideology for a moment and take a good look at what actually happened-
1) U.S (and friends, but mostly U.S) invade Iraq. Although there is heavy fighting in some areas, the vast majority of the Iraqi army did not fight at all. Although the civillian population was indeed subjected to the "shock and awe" round the clock bombing they remained mostly optimistic and hopeful to see what life without the bootheel of a dictator on their necks would be like.
2) After major combat is over, U.S and co. are the new proud occupiers of Iraq. Most of the Iraqi army and police are still intact, with only a few hard-core elements of Saddam's support having been wiped out (those of his close supporters who didn't fight went underground etc)
This was and will forever remain the high point of the U.S misadventure in Iraq. This point, shortly after the invasion was when Iraqi support for the Americans was the highest and their chances for success the greatest. In short, despite the fact that the entire war was unnecessary, the Americans managed to snatch defeat from the jaws of victory. It was what happened next that sent everything down the toilet and now more than five years later efforts in Iraq are still in damage control mode.
A) There was no strategic plan for reconstruction and security in Iraq. The commander of the U.S Army, General Shinseki openly stated and asked about this. "Don't worry about it!" he was told. He also openly stated that the biggest challenge the U.S would face would be the post-invasion security and reconstruction. His assessment that to do the job properly would take several hundred thousand troops on the ground for a prolonged period of time. In response the SECdef fired the commander of the army. The Bush Administration ignored any advice that didn't fit in with their ideological views. They believed that Iraqis would be happy just to be free and that within less than a year there would only be 20-30,000 troops in country. In terms of economic recovery, the plan was to show how great free-market economics would be by letting "private enterprise" spearhead the reconstruction. 5 years later, electrical power generation, potable water supplies and oil output in Iraq are still mostly below pre-war levels.
B) Paul Bremer. The damage this man did to the situation in Iraq can't be accurately tallied. He was Bush's man on the ground, the first real governor of Iraq after Saddam was ousted. If you want to know why the situation is so bad, look at the first two orders issued from his office- he decreed that the entire Iraqi army would be disbanded, and that any member of the Baath party had to leave their job in the administration of the country.
This turned several hundred thousand men with military training into several hundred thousand unemployed, angry young men. It was from the ranks of these fellows that provided the various resistance groups with much of their manpower. If he had kept the Iraqi army intact (most of which didn't even fight the U.S remember) and on the payroll things would have turned out very differently. The end result is the same. Today, many Sunni armed groups are no longer fighting the U.S because the U.S is paying their salaries to provide security (this is called the 'Awakening Council' if you want to research it)
De-baathification essentially gutted the country's infrastructure. In Saddam-era Iraq if you wanted to be anything, a lawyer, doctor, engineer, manager of a power plant or bank you had to be a member of the Baath party. So Bremer's de-baathification basically deprived Iraq of all it's critical staff that were keeping things running. The answer was that the roles filled by these people would then be taken over, again, by 'private enterprise' which meant foreign companies winning bids then importing workers to do jobs (very poorly) while many Iraqis with skills and education sat there unemployed and angry.
Disbanding the army and de-baathification created a MASSIVE security vacuum and basic services were no longer being looked after with the infrastructure gone (power, water, garbage removal). This was where the various militias and armed groups got a foothold and started taking over. Since the U.S wasn't looking after the people in various neighborhoods, their allegiance shifted to those would look after them.
3) The third major mistake the administration made was in refusing to acknowledge the reality that was created by their earlier decisions and actions. If they would have acknowledged the scope of their SNAFU, apologized to the Iraqi people and buckled down with proper measures they still could have reversed things. Instead Bush and his administration continued to be blinded by their ideological bias, and instead of acknowledging the reality of the situation on the ground their only answer to the fomenting resistance was to use more force to try and attain their goals.
This led to things like the Second Battle of Falujah (forgive my spelling) in which the U.S decided to make an example to the resistance by flattening half of an entire city. This led to things like Abu Ghraib in which inexperienced interrogators started resorting to methods outlawed by the geneva conventions in an effort to extract 'actionable' intelligence. This led to U.S raids in which they swept through entire neighborhoods and arrested any military age males they found or simply anyone they decided might be looking suspicious.
To his credit, Petraeus recognized the stupidity of these actions and was very much against them. That he is now the theatre commander is one of the very few things I agree with in this whole debacle.
All the while, the Administration continues to be blinded by ideological dogma. They have learned nothing and although the invasion should have never happened in the first place, they could have played this to their advantage. Instead they screw up royally and when things didn't go their way the only answer they had was to use more force. Five years later the outcome remains TBD.....