I chose "just cause." Chimpy is right, what would drive me to enlist
would be a war that I felt was necessary, although precautionary as a last resort.
Some examples, had I been alive, throughout the 20th Century the 20th Century:
Justified (relatively speaking): WW II, the Korean War, the first Gulf War, and (perhaps) the war in Afghanistan.
Unjustified (relatively speaking): WW I, the Vietnam War, and "O
I think the choice to defend while being invaded is sort of superfluos, in that you'd certainly be "justified" in defending yourself, your family, your loved ones, etc. Unless you're perhaps French.
EDIT: On a side note, I must admit that this entire thread has put B&S's song "I fought in a war" in my head.