Germany’s 1939 invasion of Poland marked the start of World War II in Europe.
Nazi Germany launched its attack on Poland on September 1, 1939, using the codename Fall Weiss. German forces attacked from the west, north, and south, while Slovakia also took part. The Soviet Union invaded eastern Poland on September 17 under the secret protocol of the Molotov–Ribbentrop Pact.
Britain and France declared war on Germany on September 3 after Germany refused to withdraw. Their declarations transformed the Polish campaign into a wider European conflict, although Poland was defeated and divided between Germany and the Soviet Union by early October.
Czechoslovakia is a common mix-up because Germany had dismantled much of that country before the war, including the occupation of Prague in March 1939. Belgium and France were invaded later, during Germany’s 1940 western offensive. The invasion of Poland is therefore the standard answer for the European beginning of World War II.