Japanese Imperialism played a role in igniting WWII because Japan wanted to "help" Asians escape western colonial rule so that the Japanese could create an empire in Asia, as soon as they arrived in Asia the U.S. withdrew certain militaristic goods and the Japanese saw this as a threat.
The U.S. not only joined the UN but they followed a new policy away from Isolationism and were now protectors of the world rather than separated from it