Leo died in 1903, and was succeeded by Pius X, whom no one would mistake for a liberal. Famous for his condemnation of “modernism,” Pius X continued the program of centralizing Church authority in an absolutist papacy. Two strategies served him well in this. One was the Oath Against Modernism, which every candidate for ordination in the world was thenceforth required to swear (it was still in force when I was ordained, in the late 1960s), and the other was a new Code of Canon Law, which would give the Pope unprecedented power over every aspect of Church life. Pacelli was one of two Vatican priests who spent more than a decade developing the code, which was finally promulgated in 1917. As Cornwell points out, Canon 218 defines the Pope’s authority as “the supreme and most complete jurisdiction throughout the Church, both in matters of faith and morals and in those that affect discipline and Church government throughout the world.”

In Europe, where the structures of Church and State were traditionally intermingled, with much overlap of political and religious authority (those schools, the appointment of those bishops), the implementation of this new Code of Canon Law required the cooperation of governments, which led to Pacelli’s next assignment. The task of negotiating treaties—concordats—that recognized the freshly claimed prerogatives of the papacy fell to him. His first success, concluded in 1914, before the code was formally published, was in Serbia, where he negotiated a concordat that served the Pope's purposes but undercut the Catholic hierarchy in Austria. “The treaty implied the abrogation,” Cornwell explains, “of the ancient protectorate rights of the Austro-Hungarian Empire over the Catholic enclave in Serbian territory.” This change in Church-State relations effectively supported Serbia’s political effort to move away from Austrian dominance. The concordat was signed on June 24, 1914. Four days later Archduke Franz Ferdinand, of Austria, was assassinated by an independence-minded Serb in Sarajevo. Cornwell comments,

The emotions prompted by the Serbian Concordat became part of the general groundswell of anti-Serbian anger.... There is no indication that Pacelli questioned the dangerous implications of the Serbian negotiations after the event. From this point of view, the episode marks the ominous beginning of Pacelli’s pattern of aloofness from the far-reaching political consequences of his diplomatic actions on behalf of the Pope.

The most important Catholic nation in Europe in the period of the First World War, even with its Protestant majority, was Germany, and in 1917, shortly after his consecration as bishop, Pacelli went to Munich as papal nuncio. Cornwell writes that his “principal task in Germany was now nothing less than the imposition, through the 1917 Code of Canon Law, of supreme papal authority over the Catholic bishops, clergy and faithful.” To that end he set out to renegotiate existing concordats with the German regional states. Ultimately he hoped for a concordat with the German nation itself, one that would solidify Vatican authority, especially in the matter of the appointment of bishops. The anti-Catholic suspicions of Protestants and Weimar liberals were not the only obstacle to the new definition of Church authority. Germany’s Catholic bishops were accustomed to holding sway in their own sphere, and the Catholic Center Party—a political organization that dated back to the nineteenth century, when it was formed to resist Otto von Bismarck’s anti-Catholic Kulturkampf—was one of the most powerful institutions in Weimar. In 1919 the Center Party drew six million votes, second only to the Social Democrats. The Center Party, occupying the crucial middle ground in the mounting chaos of the Weimar era, would provide five chancellors in the ten governments that came and went from 1919 to 1933. Catholic leaders of the party consistently rejected Pacelli’s—and the Pope’s—“urgings,” in Cornwell’s word, to stay out of coalitions with the left-wing Social Democrats. Once it was imposed on German Catholics, with the approbation of the German state, the new Code of Canon Law would end such defiance. That is the fateful background to what followed the rise of Adolf Hitler. Cornwell writes,

The acquiescence of the German people in the face of Nazism cannot be understood in its entirety without taking into account the long path, beginning as early as 1920, to the Reich Concordat of 1933; and Pacelli’s crucial role in it; and Hitler’s reasons for signing it. The negotiations were conducted exclusively by Pacelli on behalf of the Pope over the heads of the faithful, the clergy, and the German bishops.

The record compiled by Cornwell establishes, for example, that Pacelli “patently lied” to Cardinal Michael Faulhaber, the archbishop of Munich, to keep him in ignorance. “When Hitler became Pacelli’s partner in negotiations,” Cornwell observes, “the concordat thus became the supreme act of two authoritarians, while the supposed beneficiaries were correspondingly weakened, undermined, and neutralized.” The first true beneficiary was Hitler himself: the Reichskonkordat, agreed to on July 8, 1933, was his first bilateral treaty with a foreign power, and as such gave him much-needed international prestige, whether the Vatican intended it or not. (The Vatican newspaper L’Osservatore Romano published a statement on July 2 saying that the concordat implied no moral approval of Nazism, and Pacelli would make the same point later.) Yet the price Hitler demanded for the concordat was stiff—the complete withdrawal from politics (and therefore from any possible resistance to the Nazis) of all Catholics as Catholics. On July 4 the leader of the Catholic Center Party, Heinrich Brüning, who had served as Germany’s Chancellor from 1930 to 1932, had “agreed with bitterness in his heart to dissolve the party.” Hitler wanted the Center Party gone because it represented the last potential impediment to his program. It appears that Pacelli wanted it gone for the sake of his own program. (Defenders of the Vatican have argued that it did not want the Center Party dissolved, but Cornwell makes an opposite case, tied to Pacelli’s maneuvering.) That month the Center Party ceased to exist.