Within several years, renewed contact with the West profoundly altered Japanese society. The shogunate resigned, and the emperor was restored to power. The "Meiji restoration" of 1868 initiated many reforms. The feudal system was abolished, and numerous Western institutions were adopted, including a Western legal system and constitutional government along quasi-parliamentary lines.

In 1898, the last of the "unequal treaties" with Western powers was removed, signaling Japan's new status among the nations of the world. In a few decades, by creating modern social, educational, economic, military, and industrial systems, the Emperor Meiji's "controlled revolution" had transformed a feudal and isolated state into a world power.

During the 1920s, Japan progressed toward a democratic system of government. However, parliamentary government was not rooted deeply enough to withstand the economic and political pressures of the 1930s, during which military leaders became increasingly influential.

Japan invaded Manchuria in 1931 and set up the puppet state of Manchukuo. In 1933, Japan resigned from the League of Nations. The Japanese invasion of China in 1937 followed Japan's signing of the "anti-Comintern pact" with Nazi Germany the previous year and was part of a chain of developments culminating in the Japanese attack on the United States at Pearl Harbor, Hawaii, on December 7, 1941.

After almost 4 years of war, resulting in the loss of 3 million Japanese lives and the atomic bombings of Hiroshima and Nagasaki, Japan signed an instrument of surrender on the U.S.S. Missouri in Tokyo Harbor on September 2, 1945. As a result of World War II, Japan lost all of its overseas possessions and retained only the home islands. Manchukuo was dissolved, and Manchuria was returned to China; Japan renounced all claims to Formosa; Korea was occupied and divided by the U.S. and the U.S.S.R.; southern Sakhalin and the Kuriles were occupied by the U.S.S.R.; and the U.S. became the sole administering authority of the Ryukyu, Bonin, and Volcano Islands. The 1972 reversion of Okinawa completed the U.S. return of control of these islands to Japan.

After the war, Japan was placed under international control of the Allies through the Supreme Commander, Gen. Douglas MacArthur. U.S. objectives were to ensure that Japan would become a peaceful nation and to establish democratic self-government supported by the freely expressed will of the people. Political, economic, and social reforms were introduced, such as a freely elected Japanese Diet (legislature) and universal adult suffrage. The country's constitution took effect on May 3, 1947. The United States and 45 other Allied nations signed the Treaty of Peace with Japan in September 1951. The U.S. Senate ratified the treaty in March 1952, and under the terms of the treaty, Japan regained full sovereignty on April 28, 1952.