Japan's aggressive imperialism was justified. I think this because other nations would have used imperialism to control them if they didn't go after those countries before they could control over them. An example of this would be Matthew Perry, a US commodore, who sent 4 ships to Tokyo's port with a letter from Woodrow Wilson to get the Japanese to open trade. Japan replied to the letter by sending a fleet of ships to America.
A likely outcome is that World War II would have broken out earlier than on September 1, 1939. Adolf Hitler was actually preparing for war and the annexation of the Sudetenland and Czechoslovakia were just temporary moves with limited objectives. The real goal was to start war to finally achieve the great ends of the Nazi leadership: European and world domination. This is a good question for historians. Another possibility is that Germany would have to step back if it had seen determination of Britain and France not to appease it.
It is very unlikely that the Western Allies would have taken actions against the Japanese earlier, despite Japanese aggressive expansion in Manchuria and its aggression on China, as they were mainly concerned with the Nazi threat in Europe. This changed when the Japanese armies attacked and took European colonial possessions.
The European theater was influenced strongly by tank warfare on the plains of Europe. By contrast, the Pacific theater offerred very little in the way of open spaces. Fighting in the Pacific was much more reliant on infantry and was done in jungle to a large extent. In addition, the enemy was very different.
