Yes, another one. However, I'm going to endeavor to keep away from the snark and just lay down some general history here.
Long before Iraq was Iraq, there were the Shia and Sunni tribes, or Islam denominations. The differences between the two are religious and have been for a very long time. There is no love lost between the two.
Before WWI the two tribes were at something of an uneasy peace. I believe they existed within the vast Ottoman Empire so there wasn't a lot of competition for resources and the like. I'm not totally sure on this, they may have been beating on each other then as well but it was an internal matter within the Empire.
Anyway...after WWI, the Ottoman Empire was split up into the Middle East as it is now. The borders were drawm up by the western powers and were mostly drawn around oil supplies. The peoples culture was not taken into account (see Israel and Palestine for another example). This is when it all started to go to hell.
Artifical borders were placed around the Sunni and Shia and they were made to inhabit the same land. It is like taking blood rivals and sticking them in a cage match.
Iraq went through a variety of attempted monarchs and other puppet governments of Britain before the Brits gave up and a dictatorship was born. The dictators were able to keep the Sunni's and Shia's from tearing each other apart through some pretty nasty means and by regulating one tribe to second class citizens.
In steps Saddam. Saddam was a Sunni. The Sunni sect is the majority in Iraq and is indeed the largest denomination of Islam. Saddam brutally repressed the Shia minority for years.
Then we step in and kill Saddam. This is where it starts to get messy and ironic. By killing Saddam and attempting to force a democratic government in Iraq, we take the Sunnis out of power and put the Shia minority in power. This happened because the Sunnis did not believe the elections were fair and boycotted them, thus assuring themselves a minority in the new government despite being the majority of the population. Without someone to brutally enforce order the Shia and Sunnis start to shoot at each other. The Shia minority wanted revenge for being stepped on al those years and the Sunni, well who knows. I guess they didn't like being shot out or being kicked out-of-power.
Here is where the irony comes in. The Shia are more-or-less in power in Iraq. Iran is supposedly the next immenient threat. Guess what most of Iran's population is and their entire government? You guessed it, Shia. You see despite Saddam's faults, and there were many, his Sunni Iraq was a counter-balance to Shia Iran. With the Shia now in power in Iraq, and the Shia in control in Iran, and Sunni's running the show in most of the rest of the Middle East...it seems a pretty sure thing that once we leave Iraq there will be a Iran/Iraq alliance of some sort.
That is the pickle we have gotten ourselves in. The deal was done as soon as we invaded. The only thing staying in Iraq does at this point is delay the inevitable and just gets our people killed. There is no way to leave without all hell breaking loose but that was a given the moment we envaded. It is not a justification for staying there.