Actually, the simple answer is the right one. To simply turn around and say 'mankind will always be greedy and make war' is basically saying you have no trust either in your own nation or the rest of mankind to wage peace.
Considering that power and wealth are also human concepts, you are still talking about choices, you may not be able to give up your power and wealth, that is why generations in the future will suffer for it, but what you are basically implying is 'Everyone is as greedy as us, so we better get the first kick in'. Has it ever occured to you that this may not be the case, that by measuring everyone against the rod of insecurity that you seem to hold about the rest of the worlds opinions of your country, you are serving only to multiply your paranoia? Most of the world doesn't hate the US, I certainly don't, most of them just with you'd stop trying to build weapons that can kill millions of people, because lives are worth something.
It's a self-generating situation. We could stop war, but by jumping to the conclusion of 'Let's get them coz they wanna get us!', we serve only to propogate it.
Remember, Wealth and Power are as much a human-created thing as war is, just because the Romans did it 2000 years ago, doesn't pass as an excuse for humanity not to grow up.