I HATE to Say It, But...

War is actually useless in this day and age. In my opinion its a leftover evolutionary impulse.

In the past strong humans with few resources could eliminate those who help more resources and flourish making htem economically and evolutionarily successful. Good examples would be the conquistadors and the Mongolian empire. Genghis Khan holds the record for the man with the most dispersed DNA.

Humans that held onto resources and were good at holding onto them were also evolutionarily successful, like Europeans against Middle Eastern armies in the Middle Ages. The Native americans whose unforunate weakness to European pathogens made resistance impossible were nearly eliminated.

Nowadays human populations are too dense to be able to pull of a takeover. You simply cannot kill everyone of and replace yourself on their land the numbers are simply too large to deal with, and eventually any invader will be repeled. Look at the United States, we invaded Iraq and Afghanistan and instead of gaining resources we are spending them on reparing the countries we have taken over! War in our modern, money driven world is a burden. Men and women with less money do not feel secure bringing children into the world and populations start dropping. Its an unnecassary burden. The only wars that should be fought are not for control of resources, but to defend oneself.

Also if you want to reduce population you'll need to improve gyrontology to let people work longer (and be healthier) without living longer. Then the natural decline in birthrates in the Western world will be sustainable and will apropraitely lower population rates

world peace is impossible, world tolerance is attainable. that's my belief anyway. there will always be people who want nothing more than to hate and destroy, there will always be poor people who need work, and there will always be a reason to fight, that's the way of it. if there was another way to stimulate the economy the way war does i'd love to hear it. war is in our nature, there's no way of avoiding it. yes, there are people in places right now in war, or on the brink of it, but as it stands, today is the most peaceful time in centuries, melenia even.