Japan is the eleventh-most populous country in the world, as well as one of the most densely populated and urbanized. About three-fourths of the country's terrain is mountainous, concentrating its population of 126.2 million on narrow coastal plains. Japan is divided into 47 administrative prefectures and eight traditional regions. The Greater Tokyo Area is the most populous metropolitan area in the world, with more than 37.4 million residents.
Japan has been inhabited since the Upper Paleolithic period (30,000 BC), though the first mentions of the archipelago appear in Chinese chronicles from the 1st century AD. Between the 4th and 9th centuries, the kingdoms of Japan became unified under an emperor and his imperial court based in Heian-kyō. Beginning in the 12th century, however, political power was held by a series of military dictators (shōgun) and feudal lords (daimyō), and enforced by a class of warrior nobility (samurai). After a century-long period of civil war, the country was reunified in 1603 under the Tokugawa shogunate, which enacted an isolationist foreign policy. In 1854, a United States fleet forced Japan to open trade to the West, which led to the end of the shogunate and the restoration of imperial power in 1868. In the Meiji period, the Empire of Japan adopted a Western-styled constitution and pursued a program of industrialization and modernization. In 1937, Japan invaded China; in 1941, it entered World War II as an Axis power. After suffering defeat in the Pacific War and two atomic bombings, Japan surrendered in 1945 and came under a seven-year Allied occupation, during which it adopted a new constitution. Since 1947, Japan has maintained a unitary parliamentary constitutional monarchy with a bicameral legislature, the National Diet