This commemoration was introduced in the liturgical calendar by decree of Pope Pius VII on September 16, 1815, in thanksgiving for his happy return to Rome after a long and painful captivity in Savona and France due to Napoleon’s tyrannical power.
By order of Napoleon, Pius VII was arrested, 5 July, 1808, and detained a prisoner for three years at Savona, and then at Fontainebleau. In January, 1814, after the battle of Leipzig, he was brought back to Savona and set free, 17 March, on the eve of the feast of Our Lady of Mercy, the Patroness of Savona.
The journey to Rome was a veritable triumphal march. The pontiff, attributing the victory of the Church after so much agony and distress to the Blessed Virgin, visited many of her sanctuaries on the way and crowned her images (e.g. the “Madonna del Monte” at Cesena, “della Misericordia” at Treja, “della Colonne” and “della Tempestà” at Tolentino). The people crowded the streets to catch a glimpse of the venerable pontiff who had so bravely withstood the threats of Napoleon. He entered Rome, 24 May, 1814, and was enthusiastically welcomed. (McCaffrey, “History of the Catholic Church in the Nineteenth Cent.”, 1909, I, 52).
The invocation “Help of the Christians” is very old, having been included in the Litany of Loreto by Pope Saint Pius V in 1571, as a token of gratitude to the Most Holy Virgin, by virtue of Christendom’s’ victory in the famous battle of Lepanto. (Read entire article.)