 By a decree of Pope Pius VII in an apostolic letter of 1 December 1821, as part of a general reorganization of the hierarchy of Italy following the expulsion of the French, the territory of the Abbey of Guastalla became subject to the diocese of Parma.It had been subject to the diocese of Milan, under the French organization of the Cisalpine Republic (1797–1802) and the Napoleonic Kingdom of Italy (1805–1814).In his papal bull "De commisso" of 13 September 1828, Pope Leo XII, at the request of Marie Louise, Duchess of Parma, created the bishopric of Guastalla.