Short answer please
Japan, after the Meiji era and the end of the era of isolation, started import many ideas and innovations from the west. During and after the Russo-Japanese war, Japan was experiencing a wave of nationalism which spanned 4 decades. Japan began invading and occupying China, Korea and several other of their neighbors. By 1940, Japan was occupying parts of Mongolia and Russia. A succession of trade embargoes placed by the US reduced the amount of fuel and iron going to Japan, which hampered their ability to wage war in Indochina and China. At this time, however, America and many European nations were not interested in making Japan stop by force, but rather to end the conflicts in China and stop expansion via politics. Encouraged by German success in Europe, Japan was able to procure some resources, and start conquering more American and European colonies in the Pacific. Their attack on Pearl Harbor forces America into the war.