1869–1871

The First Vatican Council, the dogma of papal infallibility, and the Old Catholic schism

Pius IX convened the First Vatican Council in 1868. It opened on 8 December 1869 under the control of ultramontanists led by Cardinal Manning.

On 18 July 1870 the constitution Pastor Aeternus was adopted by an overwhelming vote: 522–533 to 2, with over a hundred abstentions. It proclaimed that the pope, speaking ex cathedra on faith and morals, is infallible. His definitions are irreformable of themselves, apart from the church's consent. The council was interrupted by the Franco-Prussian War and never formally closed.

The German historian Ignaz von Döllinger refused to accept the dogma and was excommunicated. Some Catholics in Germany, Austria, and the Netherlands formed the "Old Catholic" church under Bishop Reinkens.

On 20 September 1870, two months after the dogma's proclamation, Italian troops occupied Rome. The pope declared himself a "prisoner of the Vatican." The papacy's temporal power, dating back to the "Donation of Pepin" (756), came to an end.