World War II: Germany and Slovakia invade Poland, beginning the European phase of World War II.

World War II or the Second World War was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were tried for war crimes.

Nazi Germany, officially the German Reich and later the Greater German Reich, was the German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a totalitarian dictatorship. The Third Reich, meaning "Third Realm" or "Third Empire", referred to the Nazi claim that Nazi Germany was the successor to the earlier Holy Roman Empire (800–1806) and German Empire (1871–1918). The Third Reich, which the Nazis referred to as the Thousand-Year Reich, ended in May 1945, after 12 years, when the Allies defeated Germany and entered the capital, Berlin, ending World War II in Europe.

Slovakia, officially the (First) Slovak Republic, and from 14 March until 21 July 1939 officially known as the Slovak State, was a partially-recognized clerical fascist client state of Nazi Germany which existed between 14 March 1939 and 4 April 1945 in Central Europe. The Slovak part of Czechoslovakia declared independence with German support one day before the German occupation of Bohemia and Moravia. It controlled most of the territory of present-day Slovakia, without its current southern parts, which were ceded by Czechoslovakia to Hungary in 1938. The state was the first formally independent Slovak state in history. Bratislava was declared the capital city.