The Pope knows that the ends do not justify the means. I believe that he sincerely thinks that this action will represent a war of aggression, not to the degree of Hitler's attack of Poland in 1939, but a war of aggression nonetheless. Just War theory permits only defensive war.
Nevertheless, I don't believe that this action will represent a war of aggression. This war is going to be waged to enforce the terms of peace to which Sadam agreed after the first war. Technically, it's the same war.
Bump!