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The Interwar Period, also known as the Interbellum, was a turbulent era that spanned the years 1919-1939 between the end of World War I with the Treaty of Versailles and the start of World War II with Nazi Germany's invasion of Poland. The era may have been called "between the wars", but it was full of several wars of independence and civil wars, such as the Russian Civil War, Soviet-Polish War, Spanish Civil War, the 1920 Revolution in Iraq, the Franco-Syrian War, and the Second Italo-Abyssinian War.

While the recently-independent nations fought for their independence or underwent civil wars, several crises plagued the major powers. The Soviet Union was formed in 1922 after the Russian Empire was destroyed in a deadly civil war between the Red Army and White Army, and it established its dominance by conquering some breakaway states like Ukraine, Armenia,and Azerbaijan. With the emergence of communism, capitalism had a rival ideology that would lead to the creation of Comintern as an alliance distinct from the Allied Powers and eventually the Cold War. In addition, countries such as the Weimar Republic of Germany and the Kingdom of Italy went through revolutions by fascist ultranationalists, with the 1920s seeing street fighting between fascists and communists/Marxists in Berlin, while the 1922 March on Rome led to Benito Mussolini becoming the new dictator of Italy. The rise of fascism led to uprisings in Austria and Spain, with the latter uprising leading to the devastating Spanish Civil War that drew foreign volunteers to support Republican Spain while Fascist Italy and Nazi Germany supported Nationalist Spain with their air forces and soldiers.

In addition to uprisings and political revolutions, the era saw tensions rise between the newly-emerged great powers. Nazi Germany annexed the Sudetenland from Czechoslovakia and then the whole country in 1938, while in 1939 they annexed Austria. These crises were not met with military force out of fear of sparking a new war, but it allowed for Germany to become an even stronger country. The Soviet Union was also not met with military force as it reconquered the newly-independent states, and the Empire of Japan in the Far East took over Manchuria and formed the puppet state of Manchukuo, while also taking over a large portion of coastal China that they called Kwantung. Italy took over Abyssinia (present-day Ethiopia), and the expanding military powers of Germany, Italy, and Japan formed the "Axis Powers", a strong alliance that succeeded in providing a counterweight to the former Entente Powers of the First World War.

The division of Europe into alliance systems mirrored the causes of World War I, and some effects of World War I were still felt in the 1920s-1930s: nationalism drove countries to regain lost pride via war, and the economic crisis that became known as the "Great Depression" led to many countries seeking for opportunities to increase production; the war employed the men as soldiers and women and children as helpers on the home front, building shells, tanks, guns, bullets, planes, and other resources needed for the war. On 1 September 1939, Germany's lost pride was recovered when they began a speedy invasion of Poland, which helped their economy, satiated their desire for revenge, and proved that they had recovered from the embarrassment of World War I.