His was the shortest pontificate of the 20th Century, lasting only five years. But when Pope John XXIII died June 3, 1963, many people at that time regarded his service in the Vatican as the most memorable in modern history.

John XXIII, born Angelo Giuseppe Roncalli, was the sixth of seven pontiffs to live and die in this century. His most notable achievement was the convening in 1962 of the Second Vatican Council, or Vatican II, which resulted in major changes in the Catholic liturgy and the relationship of the Catholic Church to other religions.

On April 10. 1963, Pope John issued his encyclical "Pacem in Terris" (Peace on Earth), in which he made a dramatic plea for world peace and international cooperation. Few encyclicals from a pope have had a similar impact on international relations.

Pope John, hower, was not the first pontiff in this century to exert papal influence for world good.

There was Gioacchino Vincenzo Pecci, Pope Leo XIII. His pontificate was the longest in modern times, extending from 1878 to 1903.

Pope Leo's term in office was marked by his attempts to establish the papacy as a major influence in international relations. By most accounts, he failed, daunted by "anti-clericalism" in Italy, France and other European countries.

But Leo's arguments for the abolition of African slavery, his work to improve the Vatican's relations with Russia, and his persistent efforts to point out the dangers and costs of armed peace won for him the Papacy a new kind of respect. He died July 20. 1903.

His successor was Giuseppe Melchiore Sarto, Pope PiusX, who assumed office Aug. 9. 1903.

Pius X, who was canonized a saint in 1954, had a different focus. One of his major concerns was defending the integrity of the church against the rising tide of "modern" philosophies such as socialism.

In place of the new philosophies, Pius X urged Catholics to work together within the Church for "just and prudent" social change.

He died Aug. 20. 1914, shortly after the outbreak of World War I. The cause for his sainthood began in 1923, and was concluded successfully on May 29, 1954.

Pius X was followed by Goacomo Della Chiesa, Pope Benedict XV, whose potificate began Sept. 3, 1914. His was the second shortest pontificate of the 20th Century, lasting until 1922.

Pope Benedict spent much of his time trying, vainly, to mediate between the embattled forces in World War I. In one such effort, he sent a seven-point peace note to the combantants in August 1917.

The note called for the substitution of the "moral force of right" for the "law of material force," a simultaneous reduction of arms by all combatants, international arbitration of disputes that prompted the war, freedom of the seas, a renunciation of war debts by all involved parties, evacuation and restoration of all occupied territories, and examination "in a conciliatory spirit" of rival territorial claims.

There were no takers among the warriors. They returned polite notes, for the most part, but some accused Pope Benedict of trying to construct a "Vatican-motivated peace." The debacle of a peace effort has been described as the greatest disappointment he suffered during his pontificate.

Pope Benedict died Jan. 22, 1922. He was followed by Ambrogio Damiano Achille Ratti, Pope Pius XI.

Pius XI literally tried to take up what his predecessor could not finish. His 17-year pontificate was dedicated to achieving peace in a world greatly torn and reordered by World War I.

One of Pius XI's chief vehicles for reaching his goal was Catholic Action, an international organization of laymen whose goal was to restore society by interjecting Catholic faith and morals into public life.

That approach met with some opposition from those who viewed the church as meddling in political affairs. But Pope Pius XI persisted, while insisting that his thrust was of a purely religious character.

He died Feb. 10, 1939, and was succeeded by Eugenio Maria Giuseppe Giovanni Pacelli, Pope Pius XII.

The early years of Pius XII's 19-year pontificate were consumed by World War II and, some would argue, many of his remaining years in the papcy were overshadowed by the same event.

Pius XII worked dilligently to try to prevent World War II, but like his predecessor, Pope Benedict XV, his plea - "Nothing is lost by peace: everything is lost by war" - went largely unheard.

In the years following the war, Pius XII was accused by some of ignoring the plight of Jews trapped in Hitler's Germany. It is an accusation the church roundly denies.

His defenders say Jews, especially those in Germany, received extensive aid during Pius XII's wartime pontificate - that his financial id to Jews during that time far exceeded $4 million.

"He did not want to endanger these people," says the New Catholic Encyclopedia, in describing Pius XII's action towards Jews in World War II. "All qualified judges, even those less favorably disposed to the pope, deny that any farther papl move would have deterred Hitler from annihilating the Jews."

Pope Pius VII died Oct. 9, 1958.