"To understand why Japan lashed out, we must go back to World War I. Japan had been our ally. But when she tried to collect her share of the booty at Versailles, she ran into an obdurate Woodrow Wilson.
Wilson rejected Japan's claim to German concessions in Shantung, home of Confucius, which Japan had captured at a price in blood. Tokyo threatened a walkout if denied what she had been promised by the British. "They are not bluffing," warned Wilson, as he capitulated. "We gave them what they should not have."
In 1921, at the Washington Naval Conference, the United States pressured the British to end their 20-year alliance with Japan. By appeasing the Americans, the British enraged and alienated a proud nation that had been a loyal friend.
Japan was now isolated, with Stalin's brooding empire to the north, a rising China to the east and, to the south, Western imperial powers that detested and distrusted her.
When civil war broke out in China, Japan in 1931 occupied Manchuria as a buffer state. This was the way the Europeans had collected their empires. Yet, the West was "shocked, shocked" that Japan would embark upon a course of "aggression." Said one Japanese diplomat, "Just when we learn how to play poker, they change the game to bridge."
Japan now decided to create in China what the British had in India – a vast colony to exploit that would place her among the world powers. In 1937, after a clash at Marco Polo Bridge near Peking, Japan invaded and, after four years of fighting, including the horrific Rape of Nanking, Japan controlled the coastal cities, but not the interior."
http://www.theamericancause.org/patwhydidjapan.htm----------------------------------
In my opinion, the reason why Japan started a war of aggression was because of the League of Nations. It is mentioned that when Japan wanted to get her share of the terms promised in the Treaty Of Versailles, the other countries in the League of Nations refused to give it to her. Japan then threatened to withdraw from the League of Nations if they did not give her what was promised by the British to her. However, they still did not give her what she should have.
At this time, Japan was alone, with all the Western powers detesting and distrusting her. She probably wanted to get a revenge. Furthermore, When Japan attacked Nanking, the League of Nations did nothing to stop her, which therefore made her feel that the League of Nations could not stop her and she could do what she wanted. Therefore, Japan went on to attack the Pearl Harbor, which started off the war of aggression.
what we could have been, 11:49 PM.