Archaeological research indicates that Japan was inhabited as early as the Upper Paleolithic period. The first written mention of Japan is in Chinese history texts from the 1st century AD. Influence from other regions, mainly Imperial China, followed by periods of isolation, later from Western European influence, has characterized Japan’s history. From the 12th century until 1868, Japan was ruled by successive feudal military shoguns in the name of the Emperor. Japan entered into a long period of isolation in the early 17th century, which was only ended in 1853 when a United States fleet pressured Japan to open to the West. Nearly two decades of internal conflict and insurrection followed before the Meiji Emperor was restored as head of state in 1868 and the Empire of Japan was proclaimed, with the Emperor as a divine symbol of the nation. In the late 19th and early 20th centuries, victories in the First Sino-Japanese War, the Russo-Japanese War and World War I allowed Japan to expand its empire during a period of increasing militarism. The Second Sino-Japanese War of 1937 expanded into part of World War II in 1941, which came to an end in 1945 following the atomic bombings of Hiroshima and Nagasaki. Since adopting its revised constitution in 1947, Japan has maintained a unitary constitutional monarchy with an Emperor and an elected legislature called the National Diet.