If mainland China takes over Taiwan's sovereignty it will be a fatal blow to Japan's growing hegemony in the region, throwing into question Japan's dominance of the Asian Pacific. Japan would also have to seek political and military support from the US.
中國一旦拿到台灣主權，將對日本的霸權，產生地緣發展的致命一擊，迫使日本難以稱霸亞太，同時還要尋求美國的政治、軍事奧援。