U.S. troops could be deployed as a kind of peace-keeping force, in which case the only conceivable role for Japan would be one of logistic support. If American forces had to stay six months or a year, support would become a major undertaking. It is hard to imagine combat on the peninsula anytime in the next several years, though. Even if worst came to worst and fighting did erupt, the United States would not want the SDF to play any part. In the Americans' view, the SDF are all but useless overseas and may even be incapable of defending the homeland without U.S. help.
Some people say Japan is already a military power.
While its defense budget may seem higher than many other countries', exchange rates and high income levels render simple comparisons meaningless. Given its economic power and technological standards, theoretically Japan could arm itself heavily if it wished to, but public opinion is adamantly opposed, making it impossible for the nation to take this course in the foreseeable future. Of course it is not totally beyond the bounds of possibility that Japan might become involved in a war originating elsewhere. But nothing in the geopolitical environment suggests that there is any real possibility of this happening at least in the next several years. A more likely scenario is that some conflict would block the free flow of air and sea traffic, cutting off our lifelines.