What is becoming ever more obvious especially in Iraq is that these schisms in Islam are far more important than anything that we have caused, done, etc. You have a Sunni Arab minority that has ruled over the Shia and Kurds for generations through increasing levels of violence. And now, they find themselves out of power, and are using ever increasing levels of brutality and violence in a bid to reacquire power. But the genie is out of the bottle, and the far more numerous Shia and the Kurds now control the government, the police, and the military.
Of course, the Shia/Sunni scism in Iraq isn't the only sectarian problem in the Middle East right now. The Sunni minority in Iran is now revolting, and the almost Shia Allowite sect that rules Syria is starting to again face resistance by its Sunni majority. Of course, there is also the minority Shia in Lebannon, championed by Hizb'Allah, trying to take control over the Sunni, Druze, and Christain Lebannese.
And finally, the Saudis, protectors of the Holy Sites of Islam, etc., and rabid Wahhabi Sunnis, are in full panic mode. They thought that they had effectively encircled Shia Iran after the fall of the Shah, just to have the tables turned on them with the fall of Saddam Hussein and the rise of Shiite power in Iraq and Lebannon. Notably, Saudi nationals have apparently funded much of al Qaeda, esp. in Iraq, as well as provided many of their fighters (including, of course, Osama Ben Laden).
Of course, this is nothing new. I was reading a book on Islam today that had an extensive history running from the first revelation by the Prophet up through 9/11/01. And what is remarkable is that the Shia and the Sunnis have been fighting each other, and butchering each other, since the time of Ali, well over twelve centuries ago. Other sects, notably the Sufi, have also found themselves at the wrong end of the sword on many occasions too during the long bloody history of Islam. Indeed, it continues to this day. The Shia Ashoura pilgrimage to Karbala, the site of the death of the Prophet's grandson, Husayn ibn Ali, was bombed by (Wahhabi Sunni) al Qaeda in 2004, killing or injuring hundreds. And, maybe that is why Saddam Hussein was hung on a Sunni holy day. And it goes on.