Because of the Great Depression, many countries were trying to find ways to be able to keep their countries alive. Japan decided they needed to control more land that way the country could live on, by attacking China.
FDR "Date Which Will Live in Infamy"
Because of what Japan was doing, the U.S. decided to cut trades with Japan, no longer buying steel or oil from Japan.
Battle of Midway
Because of Japan's want for conquest of the Pacific, Japan launched an attack on Pearl Harbor because the U.S. Pacific Fleet was the only force strong enough to take on the Japanese.
Atomic Bomb on Japan
FDR's famous quote from his rallying speech after Pearl Harbor sparked the want for revenge against Japan for what had happened at Pearl Harbor.
This battle pushed Japan back and forced them to retreat causing the end of the Japanese conquest of the Pacific.
Because Japan didn't want to end the fighting after Hitler gave in, President Truman decided to launch the atomic bomb on Hiroshima, Japan and when that didn't stop Japan, Truman launched a second bomb on Nagasaki.