Hans J. Morgenthau, 76, who died Saturday at the Lenox Hill Hospital in New York City following surgery for a perforated ulcer, was a political scientist whose profoundly realistic studies of internationl affairs have had an influence that reached far beyond the academic world in which he worked.

As a scholar, Dr. Morgenthau has been credited with establishing the study of foreign policy as a separate discipline within the social sciences. Although he never held government office and although his views did not always carry the day -- he was an early and eloquent critic of U.S. policy in Vietnam -- his influence extended to the highest echelons of policy-making.

Henry A. Kissinger, President Nixon's national security adviser and then the secretary of state in the Nixon and Ford administrations, said yesterday that Dr. Morgenthau had played a major role in his own intellectual development.

"He and I continued to share the same philosophical approach to foreign policy, which did not exclude the possibility that we might disagree on specific questions," Kissinger said. "His work was seminal."

William D. Rogers, an assistant secretary and undersecretary of state in the Nixon and Ford administrations, said that what Dr. Morgenthau did "in shaping scholarship is terribly important in terms of the educational capacity for breeding future generations of diplomats."

The underpinning of Dr. Morgenthau's views was the conviction, as he put it, "that the world, imperfect as it is from the rational point of view, is the result of forces inherent in human nature." He rejected the notion that the human condition is infinitely and immediately perfectable.

From this it followed that "moral principles can never be fully realized but must at best be approximated through the ever temporary balancing of interest and the ever precarious settlement of conflicts."

Dr. Morgenthau believed that a clear perception of a nation's interests -- the basis for a sound foreign policy -- must take these general considerations into account.

At the same time, he regarded politics as a subject that required "qualitative" rather than just "quantitative" judgments. He distrusted efforts to reduce politics to data that could be fed into a computer. He also rejected such dogmatic views as holding communism to be a monolithic world conspiracy. He was perhaps influential academic in arguing that there are limits to U.S. power and to question the U.S. role as a "world policeman." He discounted "world public opinion" as a major factor in foreign policy.

In the Cold War years of the 1950s, however, he supported a strong military posture for the United States and its allies. But he also wrote:

"The anticommunist crusade as an instrument of foreign policy is likely to destroy all nations, communist and anticommunist alike."

Dr. Morgenthau's opposition to the war in Vietnam grew out of his perception that the United States had more to lose in Vietnam than it had to gain. A rule he recommended for diplomacy stated: "Never allow a weak ally to make policy decisions for you." Another of his rules was: "Never put yourself in a position from which you cannot retreat without losing face, and from which you cannot advance without great risks."

In "A New Foreign Policy for the United States," published in 1968, Dr. Morgenthau wrote that elections in South Vietnam "are not likely to settle the fundamental issue over which the war is being fought: whether South Vietnam is to be governed by a communist or noncommunist government. That issue can be decided only by a political settlement that reflects the actual distribution of military land political power between the communist and noncummunist factions."

"There are only three ways in which a government can be induced to change wrong policies: through the brutal language of the facts indicating failure, through the erosion of political support, and through the rational demonstration of error. It is that last function which political scholarship is called upon to perform. By speaking truth to power, it serves not only truth but also power."

Dr. Morgenthau's efforts to "speak truth to power" without engaging in any of the personal quarrels that characterized so much of the Vietnam era made him among the most persausive critics of the war.

Other issues on which he spoke out in criticism of U.S. policy included the overthrow of President Salvador Allende of Chile. This was in keeping with his long-held view that the United States must learn to accommodate revolutionary regimes. On the other hand, he was adamant in his condemnation of terrorism and of the treatment received by dissident elements n in the Soviet Union.

Hans Joachim Morgenthau was born in Coburg, Germany, on Feb. 17, 1904. His parents were Ludwig Morgenthau, a physician, and Frieda Bachmann Morgenthau. The family was Jewish.

The young Morgenthau was educated at the universities of Munich and Frankfurt. He practiced law in Germany from 1927 to 1930 and then became president of the Labor Court in Frankfurt. In 1932 he went to Switzerland to teach and study in Geneva. Following Hitler's accession to power in Germany in 1933, Dr. Morgenthau decided not to return to Germany. He came to the United States in 1937 after a period of teaching in Madrid.

Although he taught at several universities in the United States, Dr. Morgenthau was at the Universtiy of Chicago for most of his career in this country. From 1974 until recently, he was a professor at the New School for Social Research in New York City.

His major work was "Politics Among Nations," which grew out of his lectures at the University of Chicago and which was first published in 1948.

Dr. Morgenthau's wife, the former Irma Thormann, whom he married in 1935, died last year.

Survivors include a daughter, Susanna, of New York City, and a son, Matthew, of Chicago.