In Chapter 2 of the Bhagavad-Gita, Krishna implores Arjuna to fight this battle, telling him that it is his Holy Duty as a kshatriya (a member of the protector or warrior class) to fight. Parts of the scriptures of most religions, if taken literally, will say that there is a time to kill, notably in the Old Testament, the parts of the Qu’ran relating to jihad, and a large part of the Bhagavad-Gita, particularly in Chapter 2.
They all agree, in some way, that no killing of another human being in war is justified if not by the Authority of the Lord. War may be entered into, but only if justified by Divine cause. We humans unfortunately have taken that upon ourselves to decide when it is justified to make war. I wonder if the leaders of the world’s nations today have any idea when they hear that call. Would they know what it sounded like if they heard it? Are they too caught up in the game of war, the personal gain, the patriotic fervor they stir up by declaring war?And that’s just the thing. Some of the most destructive declarations of war are made (in the form of an act of war, not in an actual declaration) in the name of nationalist, patriotic, or religious passion (rajas), not through dharmic understanding of right (sattwa). When these rajasic things are the motivation, the war can not help but be made in the name of ego-conscious ends. This is what is called Terrorism.
In contrast, the Bhagavad-Gita is clear, concise, and personal in describing the nature of the Divine call to action, to ensure that action is taken in the name of dharma. This concept goes hand-in-hand with ahimsa (non-harmfulness) in that a truly sattwic act of war will not come unprovoked. Those who “never hit a man first” are fighting in a sattwic frame of mind. And the Pandavas in the Gita (Arjuna’s army) certainly did all they could to avoid war. The Kurus had provoked and preempted in many ways leading up to this so that the dharmic thing to do was to fight, to contain the evil.
Krishna (the Lord, Himself) delivering a one-on-one manifesto to the warrior Arjuna, recorded for all of history to reflect upon down through the ages. It is as true today as it was then, whether we are fighting a war or arguing with our neighbor.