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 Invasion of Åland was a 1918 military campaign of World War I in Åland, Finland. The islands, still hosting Soviet Russian troops, were first invaded by Sweden in late February and then by the German Empire in early March. The conflict was also related to the Finnish Civil War including minor fighting between the Finnish Whites and the Finnish Reds.