The biggest threat to Japan will be North Korea. The North Korean missile tests that have taken place over the years tend to be fired in their direction and have landed within their territory at times. Now that they have gone nuclear this will increase the risk of Japan feeling the need to make a nuclear bomb of its own. It has also prompted the US to discuss a missile defence shield that will be deployed to the area. The reasons why the North may be choosing Japan as a target are numerous. The Japanese staged a bloody occupation of the Korean peninsula during World War II that has left many Koreans with a hatred of the Japanese. They will also be doing this as they see Japan as the closest ally of the US that may be used against them if they attack South Korea. And if North Korea really is the puppet state of the Chinese that some suspect, it could be a way for the Chinese to once more hurt the Japanese without directly implicating themselves.
The Chinese and Japanese have fought numerous wars throughout history and very little love is lost between these two nations. Indeed in the past few years, under the rule of Prime Minister Koizumi Junichiro, the relationship between the two countries has been rapidly deteriorating. In recent months the new Prime Minister, who has only just come into office, has travelled on a diplomatic mission to China to try and heal these rifts but it may be too little too late if a generalised war erupts in the region. A new Sino-Japanese war could be the end result if the US is forced to use the Japanese against the North Koreans or even against the Chinese themselves in the event of an attack on Taiwan. In light of this a great militarisation is taking place in Japan as they ready themselves for the coming war.