<human>:  Why did the US drop the bombs on Japan?
<bot>: The US dropped the bombs on Japan because they were a threat to the United States. Japan had been a US ally during World War II, but had become a threat to the United States after Japan invaded China in 1937. The Japanese had also been secretly developing a nuclear bomb, and had been trying to build a delivery system. The Japanese government had been negotiating with the US, but these negotiations had stalled because the US demanded that Japan surrender, while the Japanese wanted to negotiate. On August 6, 1945, the US dropped two atomic bombs on Hiroshima and Nagasaki.