Generation Kill. While I can't know how accurate or biased it ultimately is, it gave me a much more visceral appreciation for the kind of decisions a combatant has to make, and how many of the seeming stupidities of war arise as a result of perfectly rational decisions from the various actors based on the information they have available. While I've long had an anti-war tilt, this gave me a new appreciation for how hard some goals (e.g. low civilian casualties) might be to achieve, and what some of the tradeoffs are. The psychology of the often strikingly young Marines - insightful on some subjects, simplistic on others - was also fascinating.