This guy's article in a frame:
http://www.onemarinesview.com/one_marines_view/2008/03/biggest-mistake.html
You have to be more specific though - Is the poster to encourage the people in Iraq? The soldiers in Iraq? what would be the purpose of the poster? Should it be text or a picture? What kind of design are you looking for?
But generally speaking, I'd put a picture of a US soldier protecting an Iraqi family and glorify it with words/phrases like "Freedom", "Honour", "End terrorism" and so on. Small words that mean a lot.