Interwar period

In the history of the 20th century, the interwar period, also known as the interbellum (from Latin inter bellum 'between the war[s]'), lasted from 11 November 1918 to 1 September 1939 (20 years, 9 months, 21 days) – from the end of World War I (WWI) to the beginning of World War II (WWII). It was relatively short, yet featured many social, political, military, and economic changes throughout the world. Petroleum-based energy production and associated mechanisation led to the prosperous Roaring Twenties, a time of social and economic mobility for the middle class. Automobiles, electric lighting, radio, and more became common among populations in the first world. The era's indulgences 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 WWI, and ended with the rise of fascism, particularly in Germany and Italy. China was in the midst of a half-century of instability and the Chinese Civil War between the Kuomintang, the Chinese Communist Party, and many warlords. The empires of Britain, France, and others faced challenges as imperialism was increasingly viewed negatively and independence movements emerged in many colonies; in Europe, after protracted low-level fighting most of Ireland became independent.

The Russian, Ottoman, Austro-Hungarian, and German Empires were dismantled, with the Ottoman territories and German colonies redistributed among the Allies, chiefly Britain and France. In Russia, the Bolsheviks managed to regain control of Belarus and Ukraine, Central Asia, and the Caucasus, forming the Soviet Union. 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. In the Near East, Egypt and Iraq gained independence, while Nejd conquered Hejaz, eventually becoming the Kingdom of Saudi Arabia.

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 Japanese, Italians, and Germans led to the expansion of their domains. The Soviets, similarly, harbored territorial ambitions during this period and sought to revise the post-WWI settlements.

Militarily, there was a markedly rapid advance in technology in the period, which, alongside lessons-learned from WWI, brought about strategic and tactical innovations. While the period would largely see a continuation of the development of the technologies pioneered in WWI, debates emerged as to the most effective use of these advancements. On land, discussions focused on how armoured, mechanised, and motorised forces should be employed, particularly in-relation to the traditional branches of the regular infantry, horse cavalry, and artillery. In the air, the question of allocating air forces to strategic bombing versus dedicating such forces to frontline close air support was the primary contention, with some arguing that interceptor development was outpacing bombers, and others maintaining that "the bomber will always get through." In the naval sphere, the primary question was whether battleships would maintain their dominance of the seas or be rendered virtually obsolete by naval aviation.

The interwar period ended on 1 September 1939 with the German invasion of Poland and the start of World War II. Via the events of WWII, the military deliberations and controversies characteristic of the interwar period ultimately were resolved, which served as a foundation for many of the tenets, doctrines, and strategies of modern warfare. Overall, the innovations of WWI and the interwar period would see a shift away from traditional line- and front-based warfare and towards a significantly more mobile, mechanised, and asymmetric form of combat.