According to the Japanese foreign exchange students I spoke to in 1995, the answer is that they believe the atomic bombings (of Hiroshima and Nagasaki) poisoned the land.
These foreign exchange students were college-age young men and women. The women were in secretarial schools; the men were about as bright as the women. They did not seem to be history students, so I took their answer as being typical of the culture.
I conclude that the (shocking) end of World War II forced the Japanese to consider the cultural and religious ramifications of losing a war. As a culture, the Japanese concluded that pacifism is preferable to risking further "poisoning of the land".
As Zell Faze points out, the United States imposed demilitarization upon Japan, but also set up an effective national government. Thus, Japan has not fallen into civil war. Because post-war Japan also has no land borders, and the United States set up more-or-less effective alliances to defend Taiwan, South Korea, and Japan, post-war Japan has not needed to fight foreign wars to defend itself.
It is widely assumed that Japan is capable of rapidly becoming a nuclear power. Thus, some of the logic of Mutually Assured Destruction (MAD) tends to prevent escalation of contretemps between the United States, China, Japan, and Russia (and its predecessor the Soviet Union). This also validates the Japanese public's fear of warmongering turning into nuclear devastation.