There is indeed a religious division between Sunni and Shia Islam, going back to the first generations of the religion's founding in the seventh century. You can read about those ancient religious differences and how they opened here, but the truth is that this is not terribly relevant to today's violence.You're trying to blame the Sunni/Shia feud on George W. Bush? Are you kidding me with this nonsense?
Sunni and Shia have gotten along fine for much of the Middle East's history, and the Sunni-Shia divide was just not so important for the region's politics. In the 1980s, for example, the biggest conflict in the Middle East was between two Shia-majority countries — Iran and Iraq — with Sunni powers backing Iraq.
That changed in 2003, when the United States led the invasion of Iraq to topple Saddam Hussein.
That throwaway line about Iraq being a 'Shia-majority country' in the 1980s is intended to suggest that maybe Iraq was some sort of Shi'a democracy. In fact the whole power structure was based on Sunni domination of the Shi'ites. Saddam represented a minority that ruled over a majority it feared. The intense brutality of Saddam's regime is all about the Sunni/Shia feud. His war against Iran is explicable in part because of Iran's attempts to establish and promote radical Shi'a groups that would back its own sectarian revolution. Saddam had a feud with Iran for the same reason Saudi Arabia does: because Iran is trying to export its revolution across the Middle East, using this very Sunni/Shia divide as a rallying cry.
In 2003, I attended a briefing by Physicians for Human Rights, which had accompanied British forces in the south of Iraq during and after the invasion. They had interviewed families in these heavily Shi'a regions to learn about the human rights abuses Saddam used in those areas. They reported that one in three of the households they met with had a family member who had been disappeared by the regime. This repression of Shi'ites was intense and ongoing, and in fact had gotten worse in the years running up to the war.
I think Saudi Arabia is winning, currently. Partially I think Foreign Policy is right that the oil war they're starting is going to play to their strengths against Iran's weakness. They've also managed to bring Pakistan -- the only current Islamic nuclear power -- in on their side diplomatically. Iran will have to weigh that carefully in terms of further escalations, whereas Saudi Arabia wins if it can de-escalate the crisis into a cold war fought with oil prices.
Nevertheless, the Kingdom is playing a weaker-than-expected position because the United States has suddenly changed sides. Maintaining the illusion of the "Iran deal" is so important to the Obama administration that it's ignoring Saudi Arabia's explicit call for the United States to help de-escalate the situation. The Pakistanis got asked after we ignored the invitation to play the role of big dog.