What? No love for Robocop 1? Oh, and Predator 1 with teh Ahnold is a must if it wasn't mentioned already.
And seeing all the graphic war flicks already listed, I would be remiss to not mention Braveheart and Platoon, both excellent stories despite the violence. That last one doesn't exactly glorify war or the military as other movies do, but if you are going to show war films, you might as well give some insight into the ugly BS of it too. Maybe for sometime in the future.
Also, All quiet on the Western Front (1930) for a more subtle, less than optimistic view on warfare with a sad, poetic-like ending. The 1979 version is a bit more in-your-face I think. It seems a 2012 version will be coming out as well, but I would go with the original first.