In the context of the history of the 20th century, the interwar period was the period between the end of the First World War in November 1918 and the beginning of the Second World War in September 1939.
Despite the relatively short period of time, this period represented an era of significant changes worldwide. Petroleum and associated mechanisation expanded dramatically leading to the Roaring Twenties (and the Golden Twenties), a period of economic prosperity and growth for the middle class in North America, Europe and many other parts of the world. Automobiles, electric lighting, radio broadcasts and more became commonplace among populations in the developed world. The indulgences of this era subsequently were followed by the Great Depression, an unprecedented worldwide economic downturn which severely damaged many of the world's largest economies.
Politically, this era coincided with the rise of communism, starting in Russia with the October Revolution, 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 long period of instability and civil war between the Kuomintang and the Communist Party of China. The Empires of Britain, France and others faced challenges as imperialism was increasingly viewed negatively in Europe, and independence movements in British India, French Indochina, Ireland and other regions gained momentum.
The Ottoman, Austro-Hungarian and German empires were dismantled. The Ottoman and German Empire's colonies were redistributed among the Allies. The far western part of the Russian Empire broke away: Estonia, Finland, Latvia, Lithuania and Poland became independent nations, while Bessarabia (the Republic of Moldova) chose to reunify with Romania.
The communists in the USSR managed to regain control in Ukraine, Armenia, Azerbaijan and Georgia. Ireland was split, with the larger part being independent of Britain. In the Middle East, Egypt and Iraq gained independence. During the Great Depression, Latin American countries nationalised many foreign companies (particularly American) in a bid to strengthen their local economies. Japanese, German, Italian and Soviet territorial ambitions led to expansions of these empires, which set the stage for the subsequent world war.
The German and Soviet invasion of Poland in September 1939 is considered the start of World War II and the end of the interwar period.