dyre said:
We've got better toys than we did back in the Middle Ages. If we don't stop killing each other, we'll never reach interstellar travel. You can only play with matches so many times before you get burned, and the way we're headed, even stateless terrorists will have access to a lot more than just matches.
And if we did stop killing eachother, we might never have invented nuclear fission or space travel.
A lot of Humanity's advances were invented during war or conflict.
Would there have been interest in developing a nuclear bomb if there were nobody to use it on?
Would there have been so much incentive to go into space if there wasn't an arm's race going on?
Would the Roman roads have been built if it wasn't to make deploying of Legions faster?
Would the technology level of Europe have skyrocketed above the rest of the world if there hadn't been hundreds upon hundreds of years of armed conflicts to breed competition?
I'm sure there are many more examples.
Conflict breeds invention and competition. That's just how Humanity works.
There's a reason why most civilizations(using this word in a 'Civilization The Game' sense) that strived towards Peace and Transcendence eventually got conquered and, in some cases, completely wiped out.
As an aside, I once read this short story about a space-faring civilization reaching Earth with the intent to conquer, only to, when it's all too late, realize that Human technocology in warfare was a few hundreds years advanced.
http://imgur.com/gallery/yH5KS