Don't get me wrong - Mankind should never stop trying to achieve peace on earth, as that is what keeps us reasonably civilized. But no one should ever be caught up in the belief that peace on earth could ever be a possibility - it cannot, and here is why...
The world is a natural place, and it is ruled by natural laws. For example, there are predators, and there is prey. Both are essential for the balance of nature. If not for herbivores (prey), plants would take over the world, creating an atmosphere of almost pure oxygen and depleting carbon dioxide. Such an atmosphere would spell extinction for the plants, and any other life on the planet.
To keep the herbivores in check, so they do not deplete the vegetation to the point that oxygen is no longer being created, there must be carnivores (predators) and omnivores (part carnivore, part herbivore). This creates a balance.
But nothing in nature can remain stagnant. As predators feed on an abundance of herbivores, the predator population grows, resulting in an eventual depletion of herbivores. As "prey" becomes harder to find, the predators begin to die off, as food is scarce. And as the population of predators shrinks, herbivores again proliferate. And the "life cycle" that keeps everything in balance is working as it should.
Man, an omnivore, and the only one with the power to reason, build, create and otherwise control his environment to any extent arrives on the scene. Man is a predator, but he is also prey, and in an effort to insure his ultimate survival, he must compete with other predators. So, Man uses his human abilities to "remove" other predators from the scene. Man is the ultimate predator in that respect.
And this is where it becomes impossible to have "peace on earth". Mankind does not really have any superior predator to keep his numbers in check. So the population of Man just keeps growing and growing. The only natural means for keeping the human population in check are disease and war. Without them, Man would have made himself extinct hundreds of years ago - if no one ever died in any war, the population on earth would have outstripped the planet's ability to support us centuries ago - long before technology would make it possible to feed more people with less. We would have used up all of earth's resources centuries ago, which would cause extinction of nearly all life on the planet. That is because everything that lives, consumes. For one thing to live, something else must die.
Now think about that for a moment. Imagine 12 billion people on a planet capable of sustaining only 8 billion. Since no one would willingly sit back and watch their children die of starvation, war must inevitably ensue, as people fight and kill each other for the few resources available. What would YOU do if your children were dying of starvation, and someone else had food?
Man will not stop procreating. Yet, earth's resources are finite. If one country needs energy, food, water or any other resource, that country will go to war to take what others have. It is a matter of survival.
And we all want to survive.
War, like big game hunting, helps keep populations in balance. Man has no real natural enemies other than disease, that pose a threat to our survival. So we must prey upon each other.
As Jesus said, "The poor will always be among us." There will always be "haves" and "have nots". And both will always be ready to kill in order to survive.
In short, it is the First Rule of Nature - survival of the fittest.