The encyclical Studiorum ducem, promulgated 29 June 1923, was written on the occasion of the 6th centenary of the canonization of Thomas Aquinas, whose thought is acclaimed as central to Catholic philosophy and theology.

To establish or maintain the position of the Catholic Church, Pius XI concluded a record number of concordats, including the Reichskonkordat with Nazi Germany, and he condemned their betrayals four years later in the encyclical Mit brennender Sorge ("With Burning Concern").

According to German theologian Joseph Schmidlin's Papstgeschichte der Neuesten Zeit, Benedict and Ratti repeatedly cautioned Polish authorities against persecuting Lithuanian and Ruthenian clergy.

[16] On 11 June 1921, Benedict asked Ratti to deliver his message to the Polish episcopate, warning against political misuses of spiritual power, urging peaceful coexistence with neighboring peoples, and saying that "love of country has its limits in justice and obligations".

[25] As Pius XI's first act as pope, he revived the traditional public blessing from the balcony, Urbi et Orbi ("to the city and to the world"), abandoned by his predecessors since the loss of Rome to the Italian state in 1870.

[29] Casti connubii ("Chaste Wedlock") (1930) praised Christian marriage and family life as the basis for any good society; it condemned artificial means of contraception, but acknowledged the unitive aspect of intercourse: In contrast to some of his 19th-century predecessors who favored monarchy and dismissed democracy, Pius XI took a pragmatic approach toward different forms of government.

According to an article by the historian Monsignor Vicente Cárcel y Ortí, a 1928 letter from Alfonso XIII asked the pope to restore Valencia as a cardinalitial see and appoint its archbishop, Prudencio Melo y Alcalde, a cardinal.

The Church made advances on several fronts in the 1920s, improving relations with France and, most spectacularly, settling the Roman question with Italy and gaining recognition of an independent Vatican state.

However, wrote Peter Hebblethwaite, these concordats did not prove "durable or creditable" and "wholly failed in their aim of safeguarding the institutional rights of the Church" for "Europe was entering a period in which such agreements were regarded as mere scraps of paper".

[48] Pius XI watched the rising tide of totalitarianism with alarm and delivered three papal encyclicals challenging the new creeds: against Italian Fascism Non abbiamo bisogno (1931; "We Do Not Need [to Acquaint You]"); against Nazism Mit brennender Sorge (1937; "With Deep Concern"), and against atheist Communism Divini redemptoris (1937; "Divine Redeemer").

The Pope also judged that it was folly for the French Church to continue to tie its fortunes to the unlikely dream of a monarchist restoration, and distrusted the movement's tendency to defend the Catholic religion in merely utilitarian and nationalistic terms.

[66] Ian Kershaw wrote that the Vatican was anxious to reach an agreement with the new government, despite "continuing molestation of Catholic clergy, and other outrages committed by Nazi radicals against the Church and its organisations".

The Reichskonkordat was signed by Pacelli and by the German government in June 1933, and included guarantees of liberty for the Church, independence for Catholic organisations and youth groups, and religious teaching in schools.

[71] The American National Catholic Welfare Conference wrote that Pope Pius, "again protested against the violence of the Nazis, in language recalling Nero and Judas the Betrayer, comparing Hitler with Julian the Apostate.

In March, Pope Pius XI issued the encyclical Mit brennender Sorge accusing the Nazi Government of violations of the 1933 Concordat, and of sowing the "tares of suspicion, discord, hatred, calumny, of secret and open fundamental hostility to Christ and His Church".

[83] While numerous German Catholics, including those who participated in the secret printing and distribution of the encyclical, went to jail and concentration camps, the Western democracies remained silent, which Pius XI labeled bitterly a "conspiracy of silence".

[84][85] As the extreme nature of Nazi racial anti-Semitism became obvious, and as Mussolini in the late 1930s began imitating Hitler's anti-Jewish race laws in Italy, Pius XI continued to make his position clear.

After Fascist Italy's Manifesto of Race was published, the pope said in a public address in the Vatican to Belgian pilgrims in 1938: "Mark well that in the Catholic Mass, Abraham is our Patriarch and forefather.

[90] In 1933, when the new Nazi government began to instigate its program of anti-Semitism, Pius XI ordered the papal nuncio in Berlin, Cesare Orsenigo, to "look into whether and how it may be possible to become involved" in aiding Jews.

On 10 September 1938, the Pope held a reception at Castel Gandolfo for an official delegation from Manchukuo, headed by Manchukuoan Minister of Foreign Affairs Ts'ai Yün-sheng.

[95] Mother Katharine Drexel, who founded the American order of Sisters of the Blessed Sacrament for Indians and Colored People, corresponded with Pius XI, as she had with his papal predecessors.

An emissary had shown him photos of Xavier University, a college for blacks in New Orleans which Mother Drexel had founded to provide higher education to Catholic African-Americans.

Pacelli negotiated food shipments for Russia and met with Soviet representatives, including Foreign Minister Georgi Chicherin, who rejected any kind of religious education and the ordination of priests and bishops but offered agreements without the points vital to the Vatican.

[99] Despite Vatican pessimism and a lack of visible progress, Pacelli continued the secret negotiations, until Pius XI ordered them discontinued in 1927 because they generated no results and would be dangerous to the Church if made public.

[109] In one historian's view: By the time of his death ... Pius XI had managed to orchestrate a swelling chorus of Church protests against the racial legislation and the ties that bound Italy to Germany.

[110]Pius XI planned an encyclical Humani generis unitas (The Unity of the Human Race) to denounce racism in the United States, Europe and elsewhere, as well as antisemitism, colonialism and violent German nationalism.

Aware of the implied impropriety of a Supreme Pontiff's going back on a reprimand in a matter concerning Catholic faith and morals,[116] but also deeply conscious that on a human level he had failed to keep his temper in check, he made his apology "as Achille Ratti" and in doing so stretched out his hand in friendship to Monsignor Roncalli.

[126] About half a minute before his death, Pius XI raised his right hand weakly and tried making the sign of the Cross to impart his last blessing to those gathered at his bedside.

"[126] Upon Pius XI's death, the Anglican Archbishop of Canterbury Cosmo Lang paid tribute to his efforts for world peace, calling him a man of "sincere piety" who bore his duties with exceptional "dignity and courage".

[136] In his honor, John XXIII established the Pius XI Medal that the Council of the Pontifical Academy of Sciences awards to a young scientist under 45 who has distinguished himself or herself at the international level.