The felt danger of being assaulted by the modern world pushed Pius IX to insist that the Council clearly specify the juridical primacy of the pope as far as the leadership of the Church is concerned and proclaim the infallibility of his teaching under certain conditions. After issuing Dei Filius, the dogmatic constitution against atheism, pantheism, and materialism (and making them originate from Protestantism!), the Council was ready to address the ecclesiastical issue of papal infallibility.
L'articolo 179. After 150 Years of Papal Infallibility, What? sembra essere il primo su Vatican Files.