 Pope Pius XI died on 10 February 1939 . Several historians have interpreted the conclave to choose his successor as facing a choice between a diplomatic or a spiritual candidate , and they view Pacelli 's diplomatic experience , especially with Germany , as one of the deciding factors in his election on 2 March 1939 , his 63rd birthday , after only one day of deliberation and three ballots . He was the first cardinal Secretary of State to be elected pope since Clement IX in 1667 . He was one of only two men known to have served as Camerlengo immediately prior to being elected as pope ( the other being Pope Leo XIII ) . According to rumours , he asked for another ballot to be taken to ensure the validity of his election . After his election was indeed confirmed , he chose the name Pius XII in honour of his immediate predecessor . 
