Most of the commemorative stamps issued during Pius XI's reign were tied to international conferences held at Rome. Each of these conferences was, in its own way, preoccupied with the rise of the modern authoritarian state and mounting international tensions that led to the outbreak of World War II.
The International Juridical Congress (1934, stamps issued 1935) featured papers marking the fourteenth centenary of Emperor Justinian's Corpus Iuris Civilis and the seventh centenary of Pope Gregory IX's Decretals. These documents are regarded as the basis of Western civil and ecclesiastical law, then being actively dismantled in many countries by totalitarian regimes.
The World Catholic Press Exposition (May 1936, stamps issued June 1936) sought to counteract aggression against Catholic journalists and clergy by Fascists in Germany and Italy and Communists in Russia, Mexico, and Spain.
The International Congress of Christian Archaeologists (October 1938) took place after the German occupation of Austria (Anchluss). Pius XI opened the congress with a speech that predicted a disastrous end to the Third Reich.
When Pius XI died on February 10, 1939, several values of the 1929 definitive series were overprinted with the inscription Sede Vacante ("the See is vacant") and the year 1939 in Roman numerals ("MCMXXXIX"). These stamps, issued February 18, were valid only until the election of Eugenio Cardinal Pacelli as Pope Pius XII on March 3. Thus this custom, which had been observed for centuries in papal coinage, was translated into philately for the first time.