World War I started on July 28, 1914 after the assassination of Archduke Franz Ferdinand of Austria. It ended on November 11, 1918 with an armistice with Germany. The Allies (France, Russia, Great Britain, and later the United States) fought the Central Powers, which consisted of the German Empire, Austria-Hungary, and the Ottoman Empire in a series of battles involving trench warfare and new military technology of tanks and airplanes. World War I reshaped Europe’s political geography and contributed to the Russian Revolution that turned Czarist Russia into communist Soviet Union.
The Interwar Period refers to the years between the two world wars (1918-1939). This period saw the rise of fascism in Europe and the establishment of two fascist regimes, in Italy and Germany. The Spanish Civil War (1936-1939) was a fight against Fascism. Spanish Republicans defended the democratically elected government against Spanish Nationalists who supported the military uprising led by General Franco. Franco received aid from Hitler and Mussolini. The appeasement policy of Western democracies contributed to the spread of dictatorial regimes, especially regarding Nazi Germany’s domestic repression and eventual invasion of Poland, which marked the beginning of World War II.
World War II started when Nazi Germany invaded Poland on September 1, 1939. The Axis powers, which consisted of Germany, Italy, and Japan, eventually fought against the Allied forces of the Soviet Union, the United States, Great Britain, and France. It became the bloodiest war in human history. It ended in Europe with the unconditional surrender of Germany in May 1945, and in the Pacific with the surrender of Japan in September 1945. Many American volunteers from the Spanish Civil War fought and died in World War II.
After World War II, the Soviet Union and the United States got entrenched in the Cold War, leading to a series of proxy wars after the 1950s. The Vietnam War was one of these proxy wars, with the United States trying to contain the spread of communism in Asia. It started in November 1955 and ended in April 1975 with the fall of Saigon. Many American veterans of the International Brigades opposed the U.S. involvement in the Vietnam war.