Nathan and Peter have suggested a lot of good songs, but I do have a couple to add to the mix.
The first is "Sam Stone," by John Prine, about a guy coming back from the Vietnam war. He also performs another one of my all time favorites, "Hello in There," about an old couple living in the city - it's not overtly political, but it is a fantastic song. Hopefully I'll hear them both this weekend (I'm going to see him in concert this weekend).
I would also add Billy Bragg's version of "Which Side Are You On" and "Between the Wars" to the ones Nathan mentioned.
Brett