Japan is a mountainous, volcanic island chain located in the western Pacific Ocean. The islands arc from Russia in the north toward the Korean Peninsula in the south. The country has four main islands, Hokkaido, Honshu, Shikoku and Kyushu, plus thousands of minor ones extending through the Ryukyu island chain framing the East China Sea. Japan's rugged terrain and lack of interregional connecting rivers isolated its population into separate, densely populated coastal plains. The Yamato plain, which dominates the "Inland Sea," the birthplace of Japanese civilization. The Inland Sea saw the rise of early Japanese maritime culture and facilitated communication and political control. As Japanese culture expanded over the island chain, the seat of power moved to the more productive and strategically located Kanto plain, Japan's core region and home to Tokyo, the world's largest metropolitan area. The country's primary geographic challenge is sustaining its large population on an island with little arable land and few natural resources. Japan's geography has prompted the country to alternate between periods of isolationism and expansion. When Japan unifies under strong centralized control, it is often drawn toward the continent for resources and land. This happened in the late 1500s and again in the early 1900s, leading to World War II. Given its location, the Korean Peninsula has been the corridor of invasion between Japan and China, and its status remains of strategic importance to Tokyo. The lack of natural resources in Japan continues to force the country to seek them abroad, leaving the country to balance between U.S. naval dominance and China's expanding maritime interests.