The Invasion of Poland, also known as the September Campaign or 1939 Defensive War (Polish: Kampania wrześniowa or Wojna obronna 1939 roku) in Poland and the Poland Campaign (German: Polenfeldzug) in Germany, was an invasion of Poland by Germany, the Soviet Union, and a small Slovak contingent that marked the beginning of World War II in Europe. The invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact, and ended on 6 October 1939 with Germany and the Soviet Union dividing and annexing the whole of Poland.

The morning after the Gleiwitz incident, German forces invaded Poland from the north, south, and west. As the Germans advanced, Polish forces withdrew from their forward bases of operation close to the Polish-German border to more established lines of defence to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. The two countries had pacts with Poland and had declared war on Germany on 3 September, though in the end their aid to Poland in the September campaign was very limited.

Poland ⁱ/ˈpoʊlənd/ (Polish: Polska), officially the Republic of Poland (Polish: Rzeczpospolita Polska; Kashubian: Pòlskô Repùblika; Silesian: Polsko Republika), is a country in Central Europe, bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north. The total area of Poland is 312,679 square kilometres (120,726 sq mi), making it the 69th largest country in the world and the 9th largest in Europe. Poland has a population of over 38 million people, which makes it the 34th most populous country in the world and the sixth most populous member of the European Union, being its most populous post-communist member. Poland is a unitary state made up of 16 voivodeships. Poland is a member of the European Union, NATO, the United Nations, the World Trade Organization, the Organisation for Economic Co-operation and Development (OECD), European Economic Area, International Energy Agency, Council of Europe, Organization for Security and Co-operation in Europe, International Atomic Energy Agency, G6, Council of the Baltic Sea States, Visegrád Group, Weimar Triangle and Schengen Agreement.