What was the U. S. response to North Korea's invasion of South Korea? The United States dropped an atomic bomb on North Korea.

U. S. troops prepared for attack pending UN negotiations.

The United States did nothing specific.

U. S. troops aided South Korea with support from the UN.

U.S. troops aided South Korea with support from the U.N.

Explanation:

The US troops helped with the defense of Pusan, and injected an invasion force into Incheon, which effectively cut off all supplies to the North Korean Army, and led to a complete rout to the border of China.

there should be a waiting period