Different Kind of War: The United States Army in Operation Enduring Freedom (OEF), October 2001-September 2005 [open pdf - 10MB]
From the Document: "As the sun rose on the morning of 11 September 2001, the United States (US) was at peace. American Soldiers across the country and in a number of nations across the globe woke up that day planning to conduct routine operations and training. A relatively small number of US Army units were deployed in the Balkans and the Sinai desert on peacekeeping missions. But, for most Soldiers, the day promised to be much like any other. For the Army, as well as the entire American nation, the peaceful nature of that day was shattered when just after 0900 a United Airlines jet filled with passengers plowed into the side of the North Tower of the World Trade Center in New York City. Thirty minutes later, an American Airlines jet rammed into the South Tower. While the twin towers burned, a third airliner slammed into the Pentagon in Washington, DC, and a fourth plane, possibly headed toward the US Capitol, dove straight into a field in Pennsylvania. By noon on that day, almost 3,000 people, most of whom were Americans, were dead."
US Army Combined Arms Center: http://www.usacac.army.mil/