The Invasion of Poland began on September 1, 1939, when Nazi Germany, led by Adolf Hitler, launched a military attack on Poland. This invasion marked the start of World War II in Europe. The German forces used a strategy called Blitzkrieg, which involved fast-moving and coordinated attacks by air and ground troops, overwhelming the Polish defenses. On September 17, 1939, the Soviet Union invaded Poland from the east, following a secret agreement with Germany known as the Molotov-Ribbentrop Pact. By the end of September, Poland was completely occupied, leading to significant territorial changes and the beginning of a harsh occupation for the Polish people.