War is really the only mechanism the world has left for controlling the human population. Your mortality and your inability to get along with each other combine to limit the acceleration of your otherwise infinite growth.
Predators won't do it because you exterminate them. Disease won't do it because your medical technology is too clever. Starvation won't do it because you engineer efficient food systems. Resource depletion may not even do it because you're working on alternative energy sources. But one thing can always be counted on for sure - you'll find things to fight about.
War does not determine who is right - only who is left. ~Bertrand Russell
It is forbidden to kill; therefore all murderers are punished unless they kill in large numbers and to the sound of trumpets. ~Voltaire