I haven't read all of this thread, so maybe this has been said before.
Someone has said that it is "human nature" to attack other countries just to express aggression. Come on, even animals have more of a 'just' reason for attacking members of their own species, such as sexual rivalry, food etc. Even if we assume people have a reason for starting wars, "human nature" is just a really lame excuse. I do agree that aggression is part of "human nature" and we'll probably never get rid of it. Also, aggression doesn't need to be a bad thing depending on how it is expressed or channeled. E.g. you could explain ambition as an expression of aggression in that people try to be better than others or 'fight' for a goal they want to reach. So without this sort of 'aggression' there would be no progress or competition. On the other hand, I don't understand why "human nature" is always used in a defeatist way as a label for negative aspects of human behaviour and the positive things that humankind is capable of are never mentioned. So are you saying that whatever positive things we happen to achieve are an aberration and we should actually not do them because they are not "human nature"?
To me, "human nature" also encompasses rationality and the ability to overcome aggressive impulses. So those who think wars are the only solution to problems or think it is justified to wage wars in the name of their national "interests" (whether these be material or ideological interests) are deliberately falling short of fulfilling their human potential. Humans are endowed with the ability to make decisions and if they decide to start a war, that's "fair enough" (NO, I'M NOT ARGUING IN FAVOUR OF WARS), but they don't have any right to blame it on any mysterious outside force, such as "human nature" when it was clearly their decision.