World War II: Following the start of the invasion of Poland the previous day, the Free City of Danzig (now Gdańsk, Poland) is annexed by Nazi Germany.

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.

The invasion of Poland, also known as the September Campaign, Polish Campaign, and Polish Defensive War of 1939, was a joint attack on the Republic of Poland by Nazi Germany, the Slovak Republic, and the Soviet Union, which marked the beginning of World War II. The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact. The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty.

The Free City of Danzig was a city-state under the protection and oversight of the League of Nations between 1920 and 1939, consisting of the Baltic Sea port of Danzig and nearly 200 other small localities in the surrounding areas. The polity was created on 15 November 1920 in accordance with the terms of Article 100 of the 1919 Treaty of Versailles after the end of World War I.