It goes back to when they tried to take over the world.

http://en.wikipedia.org/wiki/Article_9_of_the_Constitution_of_Japan

After their surrender during World War II and with the guidance of General Douglas MacArthur, Japan passed constituion, retaining it's tradional monarchy but government under a parlimentary democratic system. Article 9 was a section of their constitution pledging never to go to war. Here's what it says"

"Aspiring sincerely to an international peace based on order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes. In order to accomplish the aim of the preceding paragraph, land, sea and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized."

Yeah, that's all explained in the wiki, it was put there because they tried to take over the world.

"Aspiring sincerely to an international peace based on order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes. In order to accomplish the aim of the preceding paragraph, land, sea and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized."
--The Constitution of Japan: Article 9