Wolfgang Mommsen, 73, a historian who chronicled Germany's imperial past and took part in a "historians' battle" over whether Nazis crimes were unique, died Aug. 11 while swimming in the Baltic Sea off the island of Usedom. He reportedly had a heart attack.

Dr. Mommsen taught at the University of Duesseldorf, led the German Historical Institute in London from 1977 to 1985 and chaired the German Association of Historians from 1988 to 1992. For the last year, he was a fellow at the University of Erfurt in eastern Germany.

He played a role in a dispute among German academics in the 1980s about the historical context of Nazism and the Holocaust.

It was triggered by publications by several historians who challenged what they portrayed as excessive German guilt about the Holocaust, arguing that the death of 6 million Jews at the hands of the Nazis was comparable to mass killings under Soviet dictator Josef Stalin.

Dr. Mommsen and his twin brother, Hans, also a well-known historian, were in a rival camp of academics who criticized what they saw as historical revisionism.

"We had a clear position against the attempt . . . to create a new nationalism," Hans Mommsen said in a telephone interview.

Dr. Mommsen also spent much of his life researching the legacy of German sociologist and economic theorist Max Weber, the subject of his doctoral dissertation.

Dr. Mommsen was born in Marburg. His father, Wilhelm Mommsen, was a historian who was barred from teaching after World War II because he allegedly had sympathized with Nazi ideology.

One of his great-grandfathers was Theodor Mommsen, a historian who won the 1902 Nobel Prize for Literature.

In addition to his twin, Dr. Mommsen is survived by his wife and four children.