Arjuna's question in the middle of the vision is direct: tell me who you are, in this fierce form. The reply is the most famous line in the book. I am time, Krishna says, grown mature, come forth here to destroy the worlds; even without you, none of the warriors arrayed in the opposing ranks will survive. The instruction that follows is that Arjuna should therefore stand up and win glory, since these men have already been struck down, and he should be no more than the instrument.
The Sanskrit word is kala, which means both time and death, and translators have chosen between them for two centuries without settling it. The word carries the sense of duration that consumes what it measures, and the chapter has just shown the warriors of both armies rushing into blazing mouths and being ground between the teeth, which fixes the meaning past argument.
This is where the Gita answers chapter one, and the answer is harsher than the earlier chapters prepared for. Arjuna's objection was that he would be the destroyer of his family. The reply is that the destruction is already accomplished and that his participation changes only whether he acts well within it. Nothing about the deaths is denied or softened. What is denied is his ownership of them.
The passage travelled far beyond its tradition. The physicist J. Robert Oppenheimer recalled it in 1965 when describing what came to mind at the first atomic test, in a rendering, now the world, am become death, that is not quite the sentence the Sanskrit gives but which has been the line's common English form ever since.