Poland's history as a democratic nation goes back many centuries. In the sixteenth century, a Polish-led commonwealth stretched across much of central Europe and was one of the first European kingdoms to hold elections to determine its leaders. For centuries, Poland was considered one of the most politically and culturally advanced societies in Europe. However, on September 1, 1939, World War II was touched off by Hitler's Nazi Germany embarking on a massive invasion of Poland. During the war years, German and Soviet occupiers brutalized the Polish population, and establishing several of the Nazi death camps throughout the Polish landscape. Following the war, Poland became part of the Eastern Bloc. Under Communism Poland was transformed from a largely agricultural nation to an industrial center. Despite this advancement, economical mismanagement lead to increasing dissatisfaction with the Communist regime during the 1980s. Labor strikes organized by Solidarity, an independent labor union ignited widespread revolt and brought the return of democracy to Poland.