Answer: The first session of the Vatican Council I took place in St. Peter’s Basilica on December 8th, 1869. On September 20th, 1870, the Kingdom of Italy captured and occupied Rome. On October 20, 1870, Pope Pius IX indefinitely suspended the Vatican Council I. The council never resumed and it was closed formally in 1960 before the formation of the Vatican Council II. See https://vatican.com/Vatican-I/ for more details.


Centenary of the Vatican Council I
Scott 484 - 486 (1970)