When the Italian army interrupted the First Vatican Council in 1870, conquering the Papal States and with it Rome, the Council's agenda was left unfinished. Among its acts had been a dogmatic constitution on the Catholic faith (Dei Filius) – addressing the relationship of reason and faith, rejecting rationalism, materialism and fideism (blind faith), as well as a dogmatic constitution on the Church (Pastor Aeternus) – defining the extent of papal primacy and papal infallibility. While the Pope of that Council, Bl. Pius IX, considered re-opening it outside Italy, he never did, leaving the Council's unfinished business to his successors.
Fast forward to the pontificate of Pope Pius XII and we find a pontiff deeply engaged in the problems of his age, as Pius IX had been. The issues included the relationship of science, reason and faith, as well as the inner life of the Church, such as liturgical renewal and scripture studies. It is said he considered finishing the Vatican Council's work, but ultimately he never did. He, nonetheless, left a substantial body of teaching for others to build upon.
The affable, and aged, Pope John XXIII, Angelo Cardinal Roncalli elected in 1958 to succeed Pope Pius XII, was viewed by most observers as a caretaker pope, who would fill the gap between Pius XII and his ultimate successor Giovanni Cardinal Montini (the later Paul VI). Bl. Pope John XXIII, however, surprised the City and the World when on the feast of the Conversion of St. Paul, January 25th, 1959, he announced to the cardinals gathered in the Chapter Hall of the Benedictine monastery of St. Paul Outside the Walls the calling of an Ecumenical Council. As the second general council held at the Vatican, it would be known as the Second Vatican Council.