 After the outbreak of World War II, Pius XII refused to release Tisserant as the head of the Vatican Library, preventing him from returning to France to serve in the army.In 1946, Tisserant was elevated to the rank of Cardinal Bishop by Pope Pius, taking the title of Porto e Santa Rufina in 1946.After serving as vice-dean of the College of Cardinals from 1947 to 1951, he became Dean and Cardinal Bishop of Ostia on 13 January 1951.