Marilyn B. Young, a leftist, feminist, antiwar historian who challenged conventional interpretations of American foreign policy, died on Feb. 19 at her home in Manhattan, where she was a longtime professor at New York University. She was 79.

The cause was complications of breast cancer, said her son, Michael.

Professor Young’s political consciousness was rudely awakened when, as a Brooklyn teenager in 1953, she defied her father and watched from the fire escape of her family’s East Flatbush apartment as thousands of mourners gathered for the funeral of Julius and Ethel Rosenberg, who had been executed two days before at Sing Sing Prison for conspiracy to commit espionage.

“Get back inside,” her father yelled, a friend recalled. “The F.B.I. is taking pictures.”

The government’s aggressive pursuit of Soviet spies and her father’s trepidation set her on a course from which she never deviated: writing editorials for the Vassar College newspaper against red-baiting and favoring civil rights for blacks and political opportunities for women; researching a doctoral thesis that re-evaluated historic United States relations with China; and laying an anticolonial foundation for her opposition to the wars in Vietnam and Iraq.

Describing the United States as “a nation dedicated to counterrevolutionary violence,” she wrote in The New York Times Book Review in 1971 that “the most agonizing problems of recent American foreign policy have concerned not our ability to reach accommodation with acknowledged big powers, but our persistent refusal to allow revolutionary change and self-determination in smaller ones.”