Longtime Vineyard resident William P.T. Preston Jr. died April 19 at his home in Vineyard Haven. He was 85; the cause was complications following a stroke.

Mr. Preston was an historian and activist whose 1963 book Aliens and Dissenters helped open a new field of scholarly inquiry into government policies that repressed radicals and restricted civil liberties.

From 1973 to 1988, he was chairman of the history department at John Jay College of Criminal Justice in Manhattan. When he retired, he moved to the Vineyard full-time, where he had been a summer visitor since the 1950s, often visiting and staying with his aunt, Evelyn Preston and her husband, Roger N. Baldwin, at Windy Gates in Chilmark.

Born into a privileged family listed in the New York Social Register, Mr. Preston veered away from his roots after serving as a tank gunner in some of the heaviest combat of World War II. His unit, the 743d Tank Battalion, was in the first wave of U.S. forces that landed on Omaha Beach in France at H-Hour on June 6, 1944, going in ahead of the infantry in order to provide armored protection.

Thirty-two days later, Mr. Preston, then 20, was severely wounded in the battle of Normandy, leaving him partially paralyzed. The Army, he later said, “fed my identification with the underdog. . . . I learned something about class, a lot about stupidity in authority, and came out with a very resistant attitude toward higher powers.”

After the war, his emerging interest in underdogs was nourished by his uncle, Roger N. Baldwin, founder of the American Civil Liberties Union, who introduced him to prominent figures such as Alexander Meiklejohn, whose defense of free speech and first amendment rights had earned him a Time Magazine cover profile in 1928.

Aliens and Dissenters, which Mr. Preston dedicated to Mr. Baldwin, “opened up pre-McCarthy government repression as a new area of scholarly research,” according to Gerda W. Ray, an author and historian at the University of Missouri. In a Washington Post review, Supreme Court Justice William O. Douglas praised the book for revealing “the seamy side of America . . . bigotry, intolerance, hatred, suspicion of foreigners, and the use of class power to pulverize the less privileged.”

The book’s use of hitherto classified records also pointed the way to a new generation of historians and activists who began seeking access to agency files that could shed light on government surveillance, wiretapping, and suppression of information. In that sense, Aliens and Dissenters set the stage for revelations that surfaced during Watergate, in the Church committee report on CIA surveillance of domestic radicals, and the Bush administration. The book is still widely used in classrooms.

From 1954 to 1970, until he resigned in protest over the administration’s treatment of black students, Mr. Preston was professor of history and chairman of the history department at Denison University in Granville, Ohio.

In 1973, he became chairman of the history department at John Jay. He later described the job as a perfect fit for an engaged scholar. Teaching civil liberties to police officers at precinct stations provided a unique opportunity to educate those on the front lines of law enforcement about constitutional history.

He often said that many police officers were among the most receptive students he had ever had.

While at John Jay, Mr. Preston also served as chairman of the Fund for Open Information and Accountability (FOIA Inc.). The organization fought government secrecy and classification, supported the use of the Freedom of Information Act, and helped block the destruction of FBI field office files.

“He was an inspiration to generations of young scholars who turned to the study of American dissent partly because of him. He was also a key figure in opening up government archives and information to historians,” said Alice Kessler-Harris, R. Gordon Hoxie Professor of History at Columbia University.

Mr. Preston continued to write articles on the history of civil liberties after leaving John Jay, including contributing a chapter on the 1940s for the ACLU’s 50th anniversary report on its decade-by-decade accomplishments.

His account of his Omaha Beach experience appeared in the July 14, 1994, issue of the New York Review of Books. Describing the “near catastrophe” of the day, he wrote that the events “diminished forever the credibility of the concepts of strategic planning and tactical order; [they] provided me instead with a sense of chaos, random disaster and vulnerability.”

He is survived by a son, Michael Preston; three daughters, Margo Baldwin, Evie Preston and Lauren Preston-Wells; a half-brother, Dan Morgan; and five grandchildren.

A memorial service will be held in early June. In lieu of flowers, donations may be made to the American Civil Liberties Union (ACLU) in his memory.