One of the most celebrated units in the military for more than a century, by 1990, New York City's Fighting 69th Infantry Regiment of the Army National Guard was scarcely fit for duty. Its equipment was derelict, its discipline nonexistent, many of its leaders inept, and its ranks filled with kids barely out of high school who had little intention of serving their country for any longer than it took to get their paycheck, college credit, or job training. Then came the attacks of September 11 and the invasion of Iraq. In The Fighting 69th, Sean Michael Flynn, himself a member of the...