The Japanese constitution, written by the staff of General MacArthur in 1946, during the post-war US occupation, has a "non-agression" clause as its Article 9. It reconstituted its miltary as the "National Police Reserve" in 1950 and renamed it the "Self-Defense Forces" in 1954. Japan filled support roles in the Korean, Vietnam and Gulf Wars and bankrolled the Gulf War to the tune of $13 billion. Japan has taken an option to fund peacekeeping operations through the UN as an alternative to contributing soldiers. In 1992 it passed a "Peace-Keeping Law" which allowed it to contribute troops to UN missions in Cambodia, Mozambique, the Golan Heights, Rwanda and Honduras, but always in non-combat roles, constructing roads, helping run refugee camps and hospitals, etc.
Despite the its role in creating Japan's "Peace Constitution", the US has been pressuring Japan to amend Article 9 to allow full participation in combat operations. I sat in on at least 5 congressional hearings on East Timor crisis in 1999 and in every one members of congress made the statement that it's time that Japan ammended its constitution to allow for military engagement. Nevertheless, two 2000 Japanese polls showed that only 3-5% of Japanese citizens favor ammending Article 9.
To save time I'll make three points as to why Japan will not be a threat to anyone in the forseeable future:
1. The Japanese Self-Defense Forces, being rebuilt with only that role in mind, is ill-suited to make aggressive war against a formidable enemy. Modern military forces are designed in systems with specific weapons being designed to interact with other weapons for maximum effect in specific types of combat. It is no longer who has the biggest or the most guns. Japan would need to reconstitute a major portion of its military in order to try to take on another modern military,
2. The people of Japan, having suffered greatly in the dictatorial and expansionist era of Imperial Japan and World War II, have shown no interest in militarization. They have embraced democracy, and are extremely sceptical of the suggestions to take a militarily offensive posture in the world. They have no problem picking up the check, but they will never allow themselves to reconstitue the old empire or its attitudes.
3. The US and Japan are so deeply intertwined economically, and culturally that niether country would risk an act of agression against each other. Both economies, (Japan's is the second largest in the world behind ours), would be thrown into deep depressions, and the global economy would follow.
In short, we're as likely to go to war with Britain as with Japan.
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