As someone who's much better-informed about the war in Iraq than I am about the Vietnam War, it's been interesting to chart the shifting valences of the war of analogies. Initially, many war opponents were inclined to analogize Iraq to Vietnam, and conservative hawks were prepared to concede the point about Vietnam (at least ad arguendo) and dedicate their energies to the proposition that "Iraq's not another Vietnam." At some point, however, it switched and the right started making the Vietnam analogies; this time, though, using the revisionist account of Vietnam in which we were winning the war but cowardly liberals pulled the plug. Thus you started to get things like Bill Kristol waxing enthusiastic that "Bush has the good fortune of having finally found his Ulysses S. Grant, or his Creighton Abrams, in Gen. David H. Petraeus."

All of which is by way of introducing an excellent new essay on the subject by Steven Simon and Jonathan Stevenson in Democracy where they argue that, no, Vietnam wasn't winnable after all and that the impulse to keep hanging on only made the eventual outcome of the war worse than it needed to be. There's also a good brief discussion of Northern Ireland analogies up near the top.