In the history of the 20th century, the interwar period (or interbellum) lasted from 11 November 1918 to 1 September 1939 (20 years, 9 months, 21 days), the end of the First World War to the beginning of the Second World War. The interwar period was relatively short, yet featured many significant social, political, and economic changes throughout the world. Petroleum-based energy production and associated mechanisation led to the prosperous Roaring Twenties, a time of both social mobility and economic mobility for the middle class. Automobiles, electric lighting, radio, and more became common among populations in the developed world. The indulgences of the era subsequently were followed by the Great Depression, an unprecedented worldwide economic downturn that severely damaged many of the world's largest economies.

Politically, the era coincided with the rise of communism, starting in Russia with the October Revolution and Russian Civil War, at the end of World War I, and ended with the rise of fascism, particularly in Germany and in Italy. China was in the midst of half-a-century of instability and the Chinese Civil War between the Kuomintang and the Chinese Communist Party. The empires of Britain, France and others faced challenges as imperialism was increasingly viewed negatively in Europe, and independence movements emerged in many colonies; for example most of Ireland became independent after much fighting.

The Ottoman, Austro-Hungarian, and German Empires were dismantled, with the Ottoman territories and German colonies redistributed among the Allies, chiefly Britain and France. The western parts of the Russian Empire, Estonia, Finland, Latvia, Lithuania, and Poland became independent nations in their own right, and Bessarabia (now Moldova and parts of Ukraine) chose to reunify with Romania.

The Russian communists managed to regain control of the other East Slavic states, Central Asia and the Caucasus, forming the Soviet Union. Ireland was partitioned between the independent Irish Free State and the British-controlled Northern Ireland after the Irish Civil War in which the Free State fought against "anti-treaty" Irish republicans, who opposed partition. In the Middle East, both Egypt and Iraq gained independence. During the Great Depression, countries in Latin America nationalised many foreign companies (most of which belonged to the United States) in a bid to strengthen their own economies. The territorial ambitions of the Soviets, Japanese, Italians, and Germans led to the expansion of their domains.