There is a question that has not been raised in the literature on Japan's regionalism: Why does it have a strong tendency toward making the boundary of the proposed East Asian community fluid? By looking back beyond the Greater East Asia Co-Prosperity Sphere of the 1940s, a method hitherto untried, the paper shows that this Japanese propensity was also present in the first half of the twentieth century, especially in the 1920s and 1930s. Moreover, both then and now, Japan did (...) not and does not have a firm adherence to an ideology. These are two similarities between the pre-World War II period and the present (from the 1960s). On the other hand, Japan's present international situation is very different from its pre-World War II position. The paper uses the logic of the comparative method, which states that in two cases that are different in most aspects but the same in some, one or some of the similarities may explain the other similarity or similarities. It concludes that in both periods, the lack of a firm commitment to an ideology explains Japan's prejudice toward boundary fluidity. This explanation has the potential to contribute to a more comprehensive, if not yet a general theory of Japan's approach to regionalism because it applies not only to the present, but to the past as well. And it has to be stressed, the past refers not only to the Greater East Asia Co-Prosperity Sphere of the 1940s but also to the decades before. (shrink)