The culmination of events that led to World War II are generally understood to be the 1939 invasion of Poland by Nazi Germany and the 1937 invasion of the Republic of China by the Empire of Japan. These military aggressions were the decisions made by authoritarian ruling elites in Germany and Japan. World War II started after these aggressive actions were met with an official declaration of war and/or armed resistance.
In the aftermath of World War I, a defeated Germany signed the Treaty of Versailles. This caused Germany to lose a significant portion of its territory, prohibited the annexation of other states, limited the size of German armed forces and imposed massive reparations. Russia's civil war led to the creation of the Soviet Union which soon was under the control of Joseph Stalin. In Italy, Benito Mussolini seized power as a fascist dictator promising to create a "New Roman Empire." The Kuomintang (KMT) party in China launched a unification campaign against regional warlords and nominally unified China in the mid-1920s, but was soon embroiled in a civil war against its former Chinese communist allies. In 1931, an increasingly militaristic Japanese Empire, which had long sought influence in China as the first step of its right to rule Asia, used the Mukden Incident as justification to invade Manchuria; the two nations then fought several small conflicts, in Shanghai, Rehe and Hebei until the Tanggu Truce in 1933. Afterwards Chinese volunteer forces continued the resistance to Japanese aggression in Manchuria, and Chahar and Suiyuan.
German troops at the 1935 Nuremberg Rally.Adolf Hitler, after an unsuccessful attempt to overthrow the German government in 1923, became the leader of Germany in 1933. He abolished democracy, espousing a radical racially motivated revision of the world order, and soon began a massive rearming campaign. This worried France and the United Kingdom, who had lost much in the previous war, as well