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Why did the First Vatican Council suspend its work in 1870?

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Photograph of Pope Pius IX — Pius IX Pius IX is the longest-reigning pope in the history of the Roman Catholic Church. His pontificate lasted from 1846 to 1878.

The First Vatican Council suspended its work indefinitely on October 20, 1870, due to the political upheaval surrounding the Risorgimento, the movement for Italian unification.12

Following a recess that allowed council fathers to leave Rome, the situation in the city changed drastically. During the Franco-Prussian War, French troops—who had been protecting the Papal States—withdrew, leaving Rome vulnerable. On September 20, 1870, Piedmontese (Italian) troops occupied Rome, effectively ending the pope's temporal power over the region. Because of this occupation and the resulting instability, the council could not reconvene, leaving a significant portion of its planned work unfinished.213