What lies at the very heart of war? The great sages saw the cause as being a spiritual ailment. There are often symptoms of, personal ambition of a leader/leaders which often manifests as a quest for power/wealth; and the “Them and US” mentality which is a breeding ground for a lack of empathy or even hated of others.
The sages have stated that there cannot be peace without inner peace. Some of the verses I quoted are:
A person who is not disturbed by the incessant flow of desires—that enter like rivers into the ocean, which is ever being filled but is always still—can alone achieve peace, and not the man who strives to satisfy such desires. Bhagavad-gītā 2.70
One who is not connected with the Supreme can have neither transcendental intelligence nor a steady mind, without which there is no possibility of peace. And how can there be any happiness without peace? Bhagavad-gītā 2.66
A person who has given up all desires for sense gratification, who lives free from desires, who has given up all sense of proprietorship and is devoid of false ego—he alone can attain real peace. Bhagavad-gītā 2.71
Everything animate or inanimate that is within the universe is controlled and owned by the Lord. One should therefore accept only those things necessary for himself, which are set aside as his quota, and one should not accept other things, knowing well to whom they belong. Śrī Īśopaniṣad mantra 1
The humble sages, by virtue of true knowledge, see with equal vision a learned and gentle brāhmaṇa, a cow, an elephant, a dog and a dog-eater [outcaste]. Bhagavad-gītā 5.18
A person is said to be established in self-realization and is called a yogī [or mystic] when he is fully satisfied by virtue of acquired knowledge and realization. Such a person is situated in transcendence and is self-controlled. He sees everything—whether it be pebbles, stones or gold—as the same. Bhagavad-gītā 6.8
He is a perfect yogī who, by comparison to his own self, sees the true equality of all beings, in both their happiness and their distress, O Arjuna! Bhagavad-gītā 6.32
Let us move now from the practical how to the theoretical why: Why should we love our enemies? The first reason is fairly obvious. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction. So when Jesus says, "love your enemies," he is setting forth a profound and ultimately inescapable admonition. Have we not come to such an impasse in the modern world that we must love our enemies—or else? The chain reaction of evil—hate begetting hate, wars producing more wars—must be broken, or we shall be plunged into the dark abyss of annihilation. - Martin Luker King Jr.