YONAGUNI, Japan — This tiny island on Japan's western frontier has no chain convenience stores. Nature lovers can dive with hammerhead sharks and watch miniature horses graze on a hill. But the wooded mountain ranges now carry radar sites. A southern cattle ranch has been replaced with the Japanese Ground Self Defense Force's Camp Yonaguni. Japan and its ally, the United States, hold joint military exercises here. Plans are underway to add a new missile unit and expand a small airport and port.