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Why did Pope Pius IX call the First Vatican Council?

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Vatican City: St. Peter's Basilica — papacy St. Peter's Basilica, Vatican City.

Pope Pius IX convoked the First Vatican Council (1869–70) primarily to address contemporary challenges facing the Roman Catholic Church. He was deeply concerned by the rising influence of modern secular ideologies, specifically rationalism, liberalism, and materialism, which he viewed as significant dangers to the faith.12

Pius IX believed that to protect the church, it was necessary to destroy the influence of liberalism and to centralize and strengthen ecclesiastical authority. By convening the council, he sought to consolidate the church's position and clarify its doctrines in the face of these external pressures and internal calls for reform. This effort culminated in the promulgation of the dogmatic constitution Pastor Aeternus, which formally defined the papal infallibility of the pope when teaching definitively on matters of faith or morals.21