World war II/I. actually just D-day. if you lived through that you saw so many people die or have their limbs blown off you would come back a changed man.
The nature of man, as in one whole society and not the individual, cannot be changed without sever causes. For example, you have one lonesome man, you can change his nature, but you can never change the nature of man in whole.
You insert power, we as humans always have the need to break away from chains and power, we are made to be rebellious and constantly challenging opposition.
When a man gets dumped that can change a man's personality. It sure changed mine. I am much more cautious when I go into relationships and I try not to argue with anyone.
The nature of man, and all things, is survival. Plain and simple. However, within man's need for survival is greed, because we have made such a comfortable life for ourselves. Take away all he has. Give him a new motivation, stronger than his current one, whatever they may be, and man's nature will change.