It's not really an army, for those of you not in 'the know'. The country of Japan no longer has an 'Army' 'Navy' or 'Air Force' to speak of, however adverse to that, they have a collection of soldiers who make up the JSDF: Japanese Self Defence Force. It's their version of what Germany has (or did Germany finally get an actual army back?). All in all, there are about 300,000 US Soldiers on the main island of Honshu with another 70,000 on the Southern-most island of Okinawa, so in effect the American army is actually protecting Japan.
Kind of brings things into perspective as to how incredibly fucked Korea would be if they launched some low-yield nukes in Japan's direction... they would not only be killing countless numbers of Japanese, but also thousands and thousands of American's.
Needless to say, I see the USA throwing a first-full of fuck-you-up nukes in Korea's back door if that would ever happen.