I have to give plug to two books about the Mongols, which shed light on the situation in Iraq. One is The Devil's Horsemen by James Chambers and the other is Great Captains Unveiled by B.H. Liddell Hart.
Although the Mongolia is far removed from Iraq, the Mongols could travel as quickly and as far as most modern armies, which is pretty amazing when you consider that their horses were no larger than your average pony. They developed a system of fighting similar to the "blitzkrieg" that was later copied by Napoleon, Rommel, Patton, and studied intensely by Air Force Colonel John Boyd, largely credited with drafting the invasion plans for Operation Desert Storm.
In 1258, the Mongols were on the outskirts of Baghdad, which was one of the most advanced cities in the world. It housed universities, astronomical observatories, and hospitals. The Caliph (roughly a king) of the Abassid Empire, based in Baghdad, was overconfident and thought that his army was a match for the Mongols. What he failed to take into account was that his vizier (roughly second-in-command) was a Shiite Muslim, and gave the Mongols the plans to Baghdad, in the hopes that the Mongols would oust the Sunni and give him control of the city. The Mongols completely destroyed the city, and the new Shia leader only ruled for a few months before he died. Legend says he died of a broken heart, because his kingdom was reduced to rubble.
James Chambers, as he recalls this story in The Devil's Horsemen, says that there had been "some sectarian conflict" in Baghdad at the time, which is about as accurate as saying that Rosie O'Donnel "kind of likes cupcakes". Looks like things remained the same for the next eight hundred years.
Baghdad was the center of the Islamic world up until that point. The destruction of the city, which was the intellectual heartland of Islam was a catastrophic event to the Islamic world. Some even partially credit the rise of Islamic fundamentalism to the destruction of the great learning centers in Baghdad, snuffing out the voices of reason and replacing them with fundamentalist thinking.