The council opened in December 1869 with more than seven hundred bishops present and a long agenda. Infallibility was not on the original schedule; it was added by petition, and the debate over it consumed the spring of 1870. The opposition was substantial and it was not primarily doctrinal. A minority that included Bishop Felix Dupanloup of Orleans and much of the German and Austro-Hungarian episcopate argued that the definition was inopportune, that it would harden Protestant and Orthodox positions and expose Catholics in mixed societies to the charge of divided loyalty.
They lost the argument in committee and then declined to fight it on the floor. In a preliminary vote on 13 July, eighty-eight bishops voted against and sixty-two voted for it with reservations. Before the public session on 18 July, most of the minority left Rome. The final count was 433 in favour and 2 against.
What happened next was not planned by anyone in the hall. The Franco-Prussian War broke out four days later, the French garrison protecting the papal territory withdrew, and Italian troops entered Rome on 20 September. The council was suspended on 20 October and never reconvened. Its second constitution, on the bishops and on the Church as a whole, was left undrafted. That absence shaped Catholic theology for ninety years, and Vatican II's Lumen Gentium in 1964 is in large part the completion of the sentence Vatican I did not finish.