The country invaded by Nazi Germany in 1939, triggering World War II, was Poland. On September 1, 1939, Germany launched a swift military campaign known as Blitzkrieg, which marked the beginning of widespread conflict in Europe. The invasion was characterized by rapid troop movements and overwhelming air support, leading to Poland's rapid defeat. This act prompted Britain and France to declare war on Germany, thus escalating the conflict into a global war. The invasion of Poland set the stage for further military aggression across Europe.