Ralf Dahrendorf, a German sociologist whose experiences in Nazi Germany led him to develop a theory of liberalism and human freedom that often went against the grain of German politics in the postwar period, died Wednesday in Cologne. He was 80.

His death was confirmed in a statement from Chancellor Angela Merkel, who said, “Europe has lost one of its most important thinkers and intellectuals.” The cause was cancer, said his wife, Dr. Christiane Dahrendorf.

Democracy and its problems preoccupied Mr. Dahrendorf for his entire career as a scholar and as a politician in West Germany in the 1960s and 1970s. As a high school student he had been imprisoned by the Nazis for spreading leaflets opposing the regime, and early in his life he developed a deep suspicion of what he called “closed, encompassing systems.”