Arjuna appears to be worrying that all these people standing before him will be killed in the war, will be no more. Krishna tells him that that which is, has always been, and that which is not is always not.
It will be good to understand this a little.
Religion has always said this but science has also started talking in the same terms. And it is better to start with science, because whereas religion talks about the peaks that most people have not yet reached, science talks about the base, about the ground, where we are all standing.
One of the deepest findings of science is that existence cannot be taken into non-existence. That which is cannot be annihilated. And that which is not cannot be created either. All our scientific information and know-how, all the scientific laboratories of the world and all the scientists of the world together cannot annihilate even a single grain of sand. All that they can do is transform it, give it new forms.
What we call the creation of the universe, the big bang, is also nothing but the manifestation of a new form – not the creation of a new existence, only of a new form. And what we call the big crunch is not an annihilation of existence, only of the form, of the shape. The forms keep on changing, but what is hidden inside the form is unchangeable.
It is like the wheel of a cart that moves and rotates but the point at its center remains static. The wheel revolves around it. Those who only know the wheel will say that everything is a change, a flux. Those who also know the central point will say that at the center of all movement there is something unmoving, static.
And the most interesting thing is that if we separate the wheel from the axle, the wheel will not be able to move at all! The movement of the wheel depends on that which does not move. The forms change, but the change is dependent on that which is formless, which is unchanging.
When Arjuna says that all these people will die, he is talking about the form. He is saying that all these people will be annihilated: he knows about nothing beyond form. And Krishna is saying that it is not that the people you are seeing today were not there before – they were there before too: I was here before and you were here before. And it is also not that those who are here today will not be here tomorrow. No, we will be here in the future too – always, forever. Existence is from no-beginning to no-end.
So it is necessary to understand that Krishna and Arjuna are talking about two different things.