After we defeated the Japanese, we disarmed Japan and to this day they have relatively no militaristic power to speak of. In turn, we began pouring millions of dollars into Japan's economy to rebuild it.
What we now have is a country who's manufacturing facilities and economy are built for the export of goods, thus annexing much funds which would normally be needed for defense purposes.
Japan as well as the rest of the world knows that if anyone were to assault them, we would act in their defense because we would protect our investment and dirt cheap imports.
Now, the resulting concoction is some-what of a payback type relationship.
...but, I may be wrong.