Leaving out the politics involved, there have been numerous very memorable war photographs over the years. Raising of the Flag on Mount Suribachi, The Sinking of the Arizona, The Dead on the Battlefield at Gettysburg, The aftermath of a Mustard Gas attack, The Veitnamese man being executed in the street, the Soldier at on Guadacanal staring off into space, the list goes on and on.
So, what aspect of the upcomming war would you want to capture? What type of picture would you be trying to make? The massive destruction? The heroism? The effect on the civilians?
After thinking awhile, I think I'd try to document the human aspect, not heroism but suffering, weariness, horror, and the destruction of the planet and culture. Flaming oil wells, ancient art ruined, refugees, shell shocked soldiers and soldiers that are just grim but doing their duty, and, of course, the horror of death and suffering, especially if this war turns ugly.
The overall goal wouldn't be to aid any anti-war movement, but to show that war, however necessary it may sometimes be (and I really don't know personally how necessary or unnecessary it is at this point), is not to be waged lightly.