Bhagavad Gita, Chapter 11, Verse 33

Therefore do you arise and acquire fame. Conquer the enemies, and enjoy the unrivalled dominion. Verily by Myself have they been already slain; be you merely an apparent cause, O Savyasâchin (Arjuna).

Arjuna's role is redefined: he is only the apparent cause of what's coming.

Being told these warriors are already, in effect, finished shifts the weight of the coming battle. Responsibility and inevitability are shown sitting side by side, not canceling each other out.