In this work, John XXIII reacted to the political situation in the middle of the Cold War. Coming just months after the 1962 Cuban Missile Crisis, during which the Vatican served as an intermediary between the White House and the Kremlin, the document also reflected the Pope's experience of 1960 in trying to resolve difficulties arising out the four-power occupation of Berlin. The "peace encyclical" was issued only two years after the erection of the Berlin Wall. It also draws on Pope John's reading of Saint Augustines "The City of God" and Thomas Aquinas' view of Eternal Law. In this it echoes the Gospel's core values and principles of patristic and medieval thought, while reflecting the historical period in which it was written.