Japan has been inhabited since the Upper Paleolithic period (30,000 BC), though the first written mention of the archipelago appears in a Chinese chronicle finished in the 2nd century AD. Between the 4th and 9th centuries, the kingdoms of Japan became unified under an emperor and the imperial court based in Heian-kyō. Beginning in the 12th century, 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-modeled 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. Under the 1947 constitution, Japan has maintained a unitary parliamentary constitutional monarchy with a bicameral legislature, the National Diet.