War for the Pacific
Japan entered the war because the League of Nations* tried to force them to pull out of China (Which they had been invading at the time). Japan would not budge and instead they chose to attack the pacific for resources and power. America would not stand by and watch the Japanese take the pacific, so they gathered their navy in Pearl Harbor. When Japan saw this they took immediate action. The united states thought their fleet was safe because the Japanese bombers would not have enough fuel to make the flight out to pearl harbor and still get back, but Japan sent its planes anyway. Pearl Harbor was left in ruins**.
The War for the Pacific ended on 14th August 1945, after the bombing of Nagasaki and Hiroshima. For more information on the pacific war you can refer to the Timeline page, or CLICK HERE.
Image from - http://symonsez.wordpress.com/2009/12/07/pearl-harbor-attack-foretold-in-1925-carryed-out-in-war-games-in-1932-burginsaurus-comes-to-life/
*The League of Nations was devised after world war one to settle disputes and ultimately prevent another war. Obviously it wasn't very successful. For more info on the League of Nations click here http://www.historylearningsite.co.uk/leagueofnations.htm.
**For more information on Pearl Harbor Click Here