One of four children, Eugenio Pacelli was born in Rome to a family that was part of the papal, or “black,” nobility, which was devoted to service to the Vatican. His great-grandfather had served as minister of finance under Pope Gregory XVI (reigned 1831–46), his grandfather had served as undersecretary of the interior under Pius IX (1846–78), and his father was dean of the Vatican lawyers. After attending state primary schools and completing his secondary education at the Visconti Institute, Pacelli studied at the Appolinare Institute of Lateran University and Gregorian University, earning degrees in law and theology. In 1899 he was ordained a priest and in 1901 was appointed to the papal secretariat of state. He later worked under the direction of Pietro Cardinal Gasparri (Gasparri, Pietro) in preparing the new codification of canon law. He also taught international law and diplomacy at the school for papal diplomats in Rome. In 1914 Pacelli was named secretary of the Congregation for Extraordinary Affairs.

In 1917, as part of the Vatican's initiative to end World War I, Benedict XV (1914–22) named him apostolic nuncio (ambassador) to the German state of Bavaria. Pacelli enthusiastically endorsed Benedict's strict impartiality, even though the pope's attempts to mediate a peace proved unsuccessful. Following the war, he remained in the Bavarian capital, Munich, where he had a shocking experience when, during the Spartacist (Spartacus League) rising in 1919, communists burst into the papal nunciature brandishing revolvers. This encounter left an indelible impression upon Pacelli and contributed to his lifelong fear of communism. In 1920 he was dispatched as the first apostolic nuncio to the new German (Germany) Weimar Republic, with whom he sought to negotiate a concordat (a papal agreement with a national government aimed at preserving the church's privileges and freedom of action within the country in question). Pacelli's discussions with the Weimar government failed, but he succeeded in signing agreements with Bavaria in 1924 and Prussia in 1929. Moreover, by the time he departed Berlin in 1929, Pacelli was a staunch Germanophile.

He became a cardinal at the end of 1929, and early in 1930 he replaced Cardinal Gasparri as secretary of state. In 1935 he was appointed papal chamberlain (camerlengo) and thus administrator of the church during any interregnum. Pacelli and the pope who appointed him to these positions, Pius XI (1922–39), had very different personalities. While the pope was outspoken and confrontational, Pacelli was cautious and diplomatic. Yet the two complemented each other and shared the belief that church interests could better be assured by concordats (concordat)—even with regimes hostile to Christian principles—than by reliance on nation-based political parties acting on the church's behalf. In fact, Pacelli's brother Francesco helped Gasparri and Pius XI conclude the Lateran Accords (Lateran Treaty) with fascist Italy in 1929, which ended the so-called Roman Question and created the independent state of Vatican City. Pacelli, in turn, helped to negotiate concordats with Baden (1932), Austria (1933), and, controversially, with Adolf Hitler's Third Reich (July 20, 1933). Some denounced the last as an unfortunate Vatican bargain with a notorious regime.

Pacelli traveled widely on papal missions, visiting South America (1934) and North America (1936) as well as France (1935, 1937) and Hungary (1937). Because of his fluency in German and familiarity with German life, he served as Pius XI's principal adviser on Hitler and the Nazis, who assumed power in 1933. At the pope's command, Pacelli helped draft the anti-Nazi encyclical Mit brennender Sorge (“With Deep Anxiety”), written partly in response to the Nürnberg Laws and addressed to the German church on March 14, 1937. In it the papacy condemns racial theories and the mistreatment of people because of their race or nationality but does not refer to Hitler or the Nazis by name. The pope, aware of Pacelli's strong desire to prevent a break in relations between the Vatican and Berlin, commissioned the American Jesuit John La Farge to prepare an encyclical demonstrating the incompatibility of Catholicism and racism and excluded Pacelli from participating (discussed further below).

After Pius XI's death on Feb. 10, 1939, Cardinal Pacelli was elected his successor as Pope Pius XII in a short conclave. Pius XI's planned encyclical, Humani generis unitas (“The Unity of the Human Race”), against racism and anti-Semitism, was returned to its authors by the new pope. Trained as a diplomat, Pius XII followed the cautious course paved by Leo XIII and Benedict XV rather than the more confrontational one taken by Pius IX, Pius X (Pius X, Saint), and Pius XI. Hoping to serve as a “Pope of Peace,” Pius XII tried unsuccessfully to dissuade European governments from embarking on war. As part of his policy of preserving the impartiality of the Holy See and serving as mediator between nations, Pius did not want to antagonize fascist Italy and Nazi (National Socialism) Germany by issuing an encyclical that would have provoked them, a decision now cited by historians antipathetic to the pope as a sign of his indifference in the face of evil. His defenders, in turn, argue that Pius XII sought to avoid reprisals and greater harm. Whatever his motivation, when Germany invaded Poland on Sept. 1, 1939, Pius did not condemn the aggression, insisting that he had to remain above the fray, and his first encyclical, Summi pontificatus (“On the Limitations of the Authority of the State”), issued Oct. 20, 1939, reflected this diplomatic course. Pius XII, like Benedict XV, insisted that the papal position was not one of neutrality (which implied indifference) but one of impartiality. This, however, did not prevent Pius from informing the British government early in 1940 that several German generals were prepared to overturn the Nazi government if they could be assured of an honourable peace, and it did not prevent him from warning the Allies of the impending German invasion of the Low Countries in May 1940. Nor did it prevent him from futilely attempting to keep Benito Mussolini (Mussolini, Benito) from entering the war (fascist Italy joined the Axis on June 10, 1940).

Unable to stop the spread of war, Pius—the first pope to use radio extensively—made a series of Christmas broadcasts in which he returned to a number of themes raised by Benedict XV during World War I. In them he looked forward to a new world order that would supersede the selfish nationalism that had provoked the conflagration. Pius's relations with the Axis and the Allies may have been impartial, but his policies were tinged with uncompromising anticommunism. Nonetheless, despite his personal hatred of communism, he refused to support the Nazi invasion of the Soviet Union. Early in 1940, he welcomed Myron C. Taylor, U.S. President Franklin Roosevelt's personal representative to the Vatican, but did not heed Taylor's exhortations to condemn Nazi atrocities. Instead, the pope obliquely referred to the evils of modern warfare. In his Christmas message of 1942, Pius came close to revealing his sympathy for those “who without fault…sometimes only because of race or nationality, have been consigned to death or to a slow decline.” He refused to say more, fearing that public papal denunciations might provoke the Hitler regime to brutalize further those subject to Nazi terror—as it had when Dutch bishops publicly protested earlier in the year—while jeopardizing the future of the church. Although he allowed the national hierarchies to assess and respond to the situation in their countries, he established the Vatican Information Service to provide aid to, and information about, thousands of war refugees and instructed the church to provide discreet aid to Jews, which quietly saved thousands of lives. After the war, however, the pontiff was sharply criticized for not having done more to aid Hitler's victims and was seen by some as a “Pope of Silence” in the face of the Holocaust. At the same time, it was noted that Pius had much to say on subjects unrelated to the war. In his Divino afflante spiritu (“With the Help of the Divine Spirit”; 1943), for example, he sanctioned a limited use of critical historicism for biblical studies, while his Mystici corporis Christi (“Mystical Body of Christ”; 1943) sought to promote a more positive relationship between the church and nonbelievers.

As the war neared an end, the pope opposed the unconditional surrender demanded by the Allies, fearing that it would prolong the fighting and bring the Soviet Union and its communist ideology and imperium into eastern and central Europe. He also had serious reservations about the agreement reached by Roosevelt, British Prime Minister Winston Churchill (Churchill, Sir Winston), and Soviet leader Joseph Stalin (Stalin, Joseph) at the Yalta Conference and the prominent role envisioned for the Soviet Union in postwar Europe. Pius's worst fears soon materialized as Soviet domination spread and József Cardinal Mindszenty (Mindszenty, József) of Hungary and Stefan Cardinal Wyszinski of Poland were imprisoned. Fearing a communist incursion into Germany and Italy, Pius endorsed postwar western European integration and adamantly condemned the expansion of communism into eastern Europe. In 1949 he issued a decree attacking the Soviet Union's totalitarianism and authorized the Holy Office to excommunicate Catholics who joined or even collaborated with the “godless” communists.

Immediately after the war, the pope opposed the establishment of an independent political party to promote Roman Catholic and anticommunist interests in Italy; however, he eventually embraced the organization that did so, the Italian Christian Democratic Party. Indeed, the precarious postwar balance of power between the Christian Democrats and the extreme left in Italy led him to encourage the members of Catholic Action (an activist lay movement under clerical control) to participate in parliamentary politics. Thus, the Vatican openly intervened in the Italian parliamentary elections of 1948, calling upon the parish clergy and the three million members of Catholic Action organizations to support its political agenda.

Clerical intrusion in Italian public life reached another high point in the 1950s when Pius's failing health left the power of the Vatican increasingly in the hands of conservative cardinals, including Alfredo Ottaviani, head of the Holy Office. In 1952 Luigi Gedda, president of Catholic Action, fearing that the Christian Democrats (Italian Popular Party) might lose the municipal elections in Rome, proposed a Christian Democratic coalition with the parties of the right, an idea rejected by Alcide de Gasperi (De Gasperi, Alcide), the party leader and Italian prime minister, though apparently approved by the pope.

The frail and aloof pope remained an enigma for many in the 1950s. Devoted to Mary, Pius in 1950 announced the dogma that she was bodily assumed into heaven, the sole infallible declaration made after the formal proclamation of the doctrine of infallibility at the First Vatican Council (Vatican Council, First) (1869–70). For some, this posed yet another barrier to ecumenism. He also pleased conservatives but upset liberals by suppressing the French worker-priests who had been living with labourers in order to extend their ministry. He censored the questioning attitude and research priorities of the new French theology in his encyclical Humani generis (“Of the Human Race”) and supported traditional teachings regarding marital relations and birth control. On the other hand, he pleased liberals and angered conservatives by shortening and liberalizing the rules for the period of fasting before communion, taking steps to revise the liturgy, and making evening masses possible. There was also a mixed reaction from the left and right to his denunciation of atomic weaponry and his establishment of the Latin American Episcopal Council in 1956. Finally, his emphasis on the importance of the laity's role in the church and the need to reform the Curia dismayed some conservatives, who questioned his commitment to tradition. Although Pius did not hesitate to sign concordats with repressive regimes—such as one with Francisco Franco (Franco, Francisco)'s Spain in 1953 and Rafael Trujillo (Trujillo, Rafael)'s Dominican Republic in 1954—he did not allow the postwar church, much less the Vatican, to become subordinate to them—as his excommunication of Argentine strongman Juan Perón (Perón, Juan) in 1955 demonstrates.

In failing health, Pius XII died in his summer palace at Castel Gandolfo, Italy, on Oct. 9, 1958. Pius's death marked not so much the end of an era for the church as an important transition before it embarked on major reforms under John XXIII (1958–63), who convoked the Second Vatican Council (Vatican Council, Second) (1959–65). There were those who wondered what changes Pius XII would have fostered if he had convoked the council, which he had contemplated.

The controversy that followed Pius throughout his life did not stop with his death. Though upon his death he was praised effusively by world leaders and especially by Jewish groups for his actions during World War II on behalf of the persecuted, within a decade he was depicted in German playwright Rolf Hochhuth's The Deputy (1963) as indifferent to the Nazi genocide. More recently, John Cornwell's controversial book on Pius, Hitler's Pope (1999), characterized him as anti-Semitic. Both depictions, however, lack credible substantiation. Furthermore, though Pius's wartime public condemnations of racism and genocide were cloaked in generalities, he did not turn a blind eye to the suffering but chose to use diplomacy to aid the persecuted. It is impossible to know if a more forthright condemnation of the Holocaust would have proved more effective in saving lives, though it probably would have better assured his reputation. Not surprisingly, the move to beatify Pius XII alongside John XXIII in 2000 provoked a storm of controversy that may have contributed to the decision to postpone Pius's beatification.