"Four years after his [the Buddha's] attainment of enlightenment, a war took place between the city-state of Kapilavastu and that of Kilivastu over the use of water. Being told of this, [the Buddha] Sakyamuni hastened back to Kapilavastu and stood between the two great armies about to start fighting. At the sight of Sakyamuni, there was a great commotion among the warriors, who said, "Now that we see the World-Honored One, we cannot shoot the arrows at our enemies," and they threw down their weapons. Summoning the chiefs of the two armies, he asked them, "Why are you gathered here like this?" "To fight," was their reply. "For what cause do you fight?" he queried. "To get water for irrigation." Then, asked Sakyamuni again, "How much value do you think water has in comparison with the lives of men?" "The value of water is very slight" was the reply. "Why do you destroy lives which are valuable for valueless water?" he asked. Then, giving some allegories, Sakyamuni taught them as follows: "Since people cause war through misunderstanding, thereby harming and killing each other, they should try to understand each other in the right manner." In other words, misunderstanding will lead all people to a tragic end, and Sakyamuni exhorted them to pay attention to this. Thus the armies of the two city-states were dissuaded from fighting each other.
The doctrine of karma teaches that force and violence, even to the level of killing, never solves anything. Killing generates fear and anger, which generates more killing, more fear, and more anger, in a vicious cycle without end. If you kill your enemy in this life, he is reborn, seeks revenge, and kills you in the next life. When the people of one nation invade and kill or subjugate the people of another nation, sooner or later the opportunity will present itself for the people of the conquered nation to wreak their revenge upon the conquerors. Has there ever been a war that has, in the long run, really resolved any problem in a positive manner? In modern times the so-called 'war to end all wars' has only led to progressively larger and more destructive wars.
The emotions of killing translate into more and more deaths as the weapons of killing become more and more sophisticated. In prehistoric times, a caveman could explode with anger, take up his club, and bludgeon a few people to death. Now days, if, for example, the President of the United States loses his temper, who can tell how many will lose their lives as the result of the employment of our modern weaponry. And in the present we are on the brink of a global war that threatens to extinguish permanently all life on the planet. When will that happen? Perhaps when the collective selfishness of individuals to pursue their own desires --- greed for sex, wealth and power; the venting of frustrations through anger, hatred and brutal self-assertion --- overcomes the collective compassion of individuals for others, overcomes their respect for the lives and aspirations of others. Then the unseen collective pressure of mind on mind will tip the precarious balance, causing the finger, controlled ostensibly by an individual mind, to press the button that will bring about nuclear Armageddon. When the individual minds of all living beings are weighted, if peaceful minds are more predominant, the world will tend to be at peace; if violent minds are more predominant, the world will tend to be at war."
If one wants to change the world, change own being and relatives and all change.
“Sentient beings, self and others, enemies and dear ones—all are made by thoughts. It is like seeing a rope and mistaking it for a snake. When we think that the rope is a snake, we are scared, but once we see that we are looking at a rope, our fear dissipates. We have been deluded by our thoughts. Likewise, mentally fabricating self and others, we generate attachment and aversion.” ~Dilgo Khyentse Rinpoche