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. 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 Russia, the Bolsheviks managed to regain control of Belarus and Ukraine, Central Asia, and the Caucasus, forming the Soviet Union. In the Near East, 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 Japanese, Italians, and Germans led to the expansion of their domains. The Soviets additionally harbored territorial ambitions during this period and would seek to revise the post-WWI settlements.

Militarily, the period would see a markedly rapid advance in technology which, alongside lessons learned from WWI, would catalyse new 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 military deliberations and controversies characteristic of the interwar period would ultimately find resolution via the events of WWII, 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.

Following the Armistice of Compiègne on 11 November 1918 that ended the fighting of World War I, the years 1918–1924 were marked by turmoil as the Russian Civil War continued to rage on, and Eastern Europe struggled to recover from the devastation of the First World War and the destabilising effects of not just the collapse of the Russian Empire, but the destruction of the German, Austro-Hungarian, and Ottoman Empires, as well. There were numerous new or restored countries in Southern, Central, and Eastern Europe, some small in size, such as Lithuania and Latvia, and some larger, such as Poland and the Kingdom of Serbs, Croats, and Slovenes. The United States gained dominance in world finance. Thus, when Germany refused to pay further war reparations to Britain, France and other former members of the Entente, the Americans came up with the Dawes Plan and Wall Street invested heavily in Germany, which repaid its reparations to nations that, in turn, used the dollars to pay off their war debts to Washington. By the middle of the decade, prosperity was widespread, with the second half of the decade known as the Roaring Twenties.

The important stages of interwar diplomacy and international relations included resolutions of wartime issues, such as reparations owed by Germany and boundaries; American involvement in European finances and disarmament projects; the expectations and failures of the League of Nations; the relationships of the new countries to the old; the distrustful relations of the Soviet Union to the capitalist world; peace and disarmament efforts; responses to the Great Depression starting in 1929; the collapse of world trade; the collapse of democratic regimes one by one; the growth of efforts at economic autarky; Japanese aggressiveness toward China, occupying large amounts of Chinese land, as well as border disputes between the Soviet Union and Japan, leading to multiple clashes along the Soviet and Japanese occupied Manchurian border; fascist diplomacy, including the aggressive moves by Mussolini's Italy and Hitler's Germany; the Spanish Civil War; Italy's invasion and occupation of Abyssinia (Ethiopia) in the Horn of Africa; the appeasement of Germany's expansionist moves against the German-speaking nation of Austria, the region inhabited by ethnic Germans called the Sudetenland in Czechoslovakia, the remilitarisation of the League of Nations demilitarised zone of the German Rhineland region, and the last, desperate stages of rearmament as the Second World War increasingly loomed.

Disarmament was a very popular public policy. However, the League of Nations played little role in this effort, with the United States and Britain taking the lead. U.S. Secretary of State Charles Evans Hughes sponsored the Washington Naval Conference of 1921 in determining how many capital ships each major country was allowed. The new allocations were actually followed and there were no naval races in the 1920s. Britain played a leading role in the 1927 Geneva Naval Conference and the 1930 London Conference that led to the London Naval Treaty, which added cruisers and submarines to the list of ship allocations. However the refusal of Japan, Germany, Italy and the USSR to go along with this led to the meaningless Second London Naval Treaty of 1936. Naval disarmament had collapsed and the issue became rearming for a war against Germany and Japan.