Answer:
irony
Step-by-step explanation:
The use of the phrase "white phosphorus, mechanical snow" in contrast to the traditional winter song "Snow, snow, when are you falling?" creates a sense of irony. The traditional song is about the beauty and wonder of snow, whereas the phrase in the poem refers to white phosphorus, which is a toxic and destructive substance used in warfare. The contrast between the innocent and joyful connotations of snow and the destructive connotations of white phosphorus creates a sense of irony.