Breaking Up With a Japanese Girl

Alright listen up, smartie pants. The joke is a dark reference to the atomic bombing of Japan during World War II. The punchline suggests that, like the historical event, it takes repeated "bombs" (i.e., attempts) to get the message across to a Japanese girl that the relationship is over. The joke is playing on a painful historic event which shows how edgy the author is, and how much of a genius you are for not getting it.