Pope Pius XI (Latin: Pius PP. XI; Italian: Pio XI; 31 May 1857 – 10 February 1939), born Ambrogio Damiano Achille Ratti, was Pope from 6 February 1922, and sovereign of Vatican City from its creation as an independent state on 11 February 1929 until his death on 10 February 1939. He issued numerous encyclicals including Quadragesimo Anno, highlighting capitalistic greed of international finance, and social justice issues, and Quas Primas, establishing the feast of Christ the King. He took as his papal motto, "Christ's peace in Christ's kingdom".

Achille Ratti was an accomplished scholar, librarian and priest. He celebrated his 60th birthday as a priest on 31 May 1917. Fewer than five years later, on 6 February 1922, he was elected Pope, succeeding Pope Benedict XV, who was only thirty months older and thus from the same generation as Ratti. In those five years, he had short stints as papal nuncio in Poland, which forced him to leave the country, and as Archbishop of Milan and Cardinal-Priest of Ss. Silvestro e Martino ai Monti, where he served for a few months before being elected Pope. He chose the name Pius, and his personality was strong, similar to Pius IX and Pius X. But as a scholar, he was open to science and research like no other Pope since Leo XIII.