sep 7, 1931 - Japanese expansion in the Pacific
After the stock market crashed, the US decided to go into a state of isolation so they could focus on improving themselves instead of worrying about other countries. However, the Japanese pursued policy of expansion due to population growth and need for larger markets for products. They refused to ignore the Untied Nations (UN) orders to return from Manchuria. These events caused alarm in the US and the isolation quickly came to an end. The Jpanaese starting to refuse orders worried the United States, giving them a reason to join the war.
Added to timeline:
Causes of US Entering WWII-Meaghan and Felicia