"A word of explanation about how the information in this book was obtained, evaluated and used. This book is designed to present, as best my reporting could determine, what really happened. The core of this book comes from the written record--National Security Council meeting notes, personal notes, memos, chronologies, letters, PowerPoint slides, e-mails, reports, government cables, calendars, transcripts, diaries and maps. Information in the book was supplied by more than 100 people involved in the Afghanistan War and national security during the first 18 months of President Barack Obama's administration. Interviews were conducted on "background," meaning the information could be used but the sources would not be identified by name. Many sources were interviewed five or more times. Most allowed me to record the interviews, which were then transcribed. For several sources, the combined interview transcripts run more than 300 pages. I have attempted to preserve the language of the main characters and sources as much as possible, using their words even when they are not directly quoted, reflecting the flavor of their speech and attitudes. Many key White House aides were interviewed in-depth. They shared meeting notes, important documents, recollections of what happened before, during and after meetings, and assisted extensively with their interpretations. Senior and well-placed military, intelligence and diplomatic officials also provided detailed recollections, read from notes or assisted with documents. Since the reporting was done over 18 months, many interviews were conducted within days or even hours after critical discussions. This often provided a fresher and less-calculated account. Dialogue comes mostly from the written record, but also from participants, usually more than one. Any attribution of thoughts, conclusions or feelings to a person was obtained directly from that person, from notes or from a colleague whom the person told. Occasionally, a source said mid-conversation that something was "off-the-record," meaning it could not be used unless the information was obtained elsewhere. In many cases, I was able to get the information elsewhere so that it could be included in this book. Some people think they can lock up and prevent publication of information by declaring it "off-the-record" or that they don't want to see it in the book. But inside any White House, nearly everyone's business and attitudes become known to others. And in the course of multiple, extensive interviews with firsthand sources about key decision points in the war, the role of the players became clear. Given the diversity of sources, stakes and the lives involved, there is no way I could write a sterilized or laundered version of this story. I interviewed President Obama on-the-record in the Oval Office for one hour and 15 minutes on Saturday, July 10, 2"