Yes, it is well time that all wars were stopped and even better that no wars were created, but as you say, it is part of the human make up, but may not be because of stupidity, but that, in some instances could be true.
No, I feel the major problem is envy, the wanting of something that is not yours, or in fact ego, so the human psyche is the main problem.
Also power corrupts, as many feel they have the power to overcome or ignore rulings.
Humans will not change, so what can be done, well be very careful who you elevate into positions of power and have a robust removal process available when it is clear that abuses of power are occurring.
But persons in these situation will be adept at fooling others by deceptions and there will always be persons who can easily be deceived.