World War I radically changed the political map of Europe with the defeat of the Central Powers (Austria-Hungary, Germany, Bulgaria, and the Ottoman Empire) and the Bolsheviks seizing power in Russia in 1917 and establishing the Soviet Union. Meanwhile, the victorious Allied Powers such as France, Belgium, Italy, Romania and Greece gained new territory as a result of the war, and new nation states emerged from the collapse of the Austro-Hungarian, Ottoman and Russian Empires.

The League of Nations assembly meeting in Geneva, 1930
The League of Nations was founded in 1919 during the Paris Peace Conference to prevent a potential future world war. The main objectives of the organization were to prevent armed conflict through collective security, disarmament, and resolve international disputes through peaceful negotiations and arbitration.

Despite the strong pacifist views that emerged after World War I,irredentism and revanchist nationalism began to develop in many European countries during the same period. These views were particularly evident in Germany due to the significant territorial, colonial and economic losses imposed by the Versailles Peace Treaty. Due to the treaty, Germany lost 13% of its territory in Europe and all of its overseas territories, was prohibited from annexing other states, was forced to pay reparations, and placed limits on the size and equipment of the country's armed forces.

With the German Revolution of 1918-1919, the German Empire collapsed and a democratic government was established, later known as the Weimar Republic. The interwar period saw conflict between supporters of the newly established republic and hardline opponents on both the right and left. Italy, an Entente ally, made some post-war territorial gains However, Italian nationalists were angry that the promises made by the United Kingdom and France to ensure Italian entry into the war had not been fulfilled in the peace treaty. From 1922 to 1925, the fascist movement led by Benito Mussolini; He seized power in Italy with a nationalist, totalitarian and class collaborationist ideology that abolished representative democracy, put pressure on socialist, left and liberal forces, and pursued an expansionist foreign policy that promised to make Italy a great power and create a "New Roman Empire" .

Adolf Hitler at a Nazi political rally in Nuremberg, August 1939
Adolf Hitler became Chancellor of Germany in 1933 after a failed coup attempt to overthrow the German government in 1923. In his book Mein Kampf, Hitler stated that their only goal was not to repeal the Treaty of Versailles; He explained that his goals included defeating communism and ensuring the dominance of the German race throughout the world on a European basis. After coming to power, he abolished democracy and soon launched a massive rearmament effort. Stalin, who frequently criticized the West until Hitler became chancellor, joined the League of Nations in 1934 after Hitler became chancellor and began to advocate international collective security. After Stalin's moves, the United States sent some delegations to Moscow. Fearing Nazi expansion, France also took various steps to establish relations with the Soviet Union.

Meanwhile, in order to secure its alliance, France turned a blind eye to Italy's invasion of Ethiopia, which it wanted as a colony. The situation worsened with the legal reunification of the Saar Region with Germany in early 1935, and Hitler violating the Treaty of Versailles by accelerating his rearmament program and introducing conscription.

Romania, whose territory was expanding, and the newly established Poland formed an alliance. (The alliance was broken by Romania when Nazi Germany declared war on Poland.)

To contain Germany, the United Kingdom, France, and Italy signed the Treaty of Stresa in April 1935, an important step towards military globalization; however, in June of the same year, an independent maritime treaty was signed between the United Kingdom and Germany, easing previous restrictions. The Soviet Union, which became concerned about Germany's aims to invade large areas of Eastern Europe, prepared a mutual assistance agreement with France. However, the Franco-Soviet pact had to go through the League of Nations bureaucracy before it could come into force, making the treaty ineffective. The USA, which was closely interested in the events in Europe and Asia, approved the Neutrality Laws in August of the same year.

In March 1936, Hitler violated the Treaties of Versailles and Locarno by calling the Soviet Union–France agreements a "red threat" and rearming the Rhineland, facing little international opposition for his policy of appeasement.In October 1936, Germany and Italy formed the Rome-Berlin Axis. A month later, Germany and Japan signed the Anti-Comintern Pact, which Italy joined the following year

Asia
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The Kuomintang party in China launched a unification campaign against regional warlords and unified China in the mid-1920s; However, a civil war soon broke out in the country against their former allies, the Chinese Communist Party and newly emerged regional warlords. In 1931, the increasingly militaristic Empire of Japan, which had long sought an opportunity to gain influence in China because its government saw the country's right to rule Asia,used a pseudo-state as an excuse to invade Manchuria and establish the puppet state of Manchukuo. He carried out the flag operation Mukden Incident.

China appealed to the League of Nations to stop Japan's invasion of Manchuria. Countries such as the United States and the United Kingdom were dealing with the effects of the Great Depression and could not impose military sanctions on this invasion of Japan, while the Soviet Union under Stalin did not want to spoil relations with Japan. The only sanction for Japan's invasion was condemnation. Japan withdrew from the League of Nations after the condemnation decision. The two countries later clashed in several battles in Shanghai, Rehe, and Hebei until the Tanggu Truce was signed in 1933. Chinese volunteers later continued resistance against Japanese aggression in Manchuria, Chahar, and Suiyuan. After the Xi'an Incident in 1936, the Kuomintang and communist forces agreed to a ceasefire to fight together against Japan.