The papal conclave held from 2 to 28 September 1823 ended with the election of Annibale della Genga to succeed the death recently deceased Pius VII as pope. Della Genga took the name Leo XII.

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Papal conclave September 1823
Dates and location
2–28 September 1823 Quirinal Palace, Papal States
Key officials
Dean	Giulio Maria della Somaglia
Sub-dean	Bartolomeo Pacca
Camerlengo	Bartolomeo Pacca
Protopriest	Giuseppe Firrao
Protodeacon	Fabrizio Ruffo Dionigi
Election
Vetoed	Antonio Gabriele Severoli
Elected pope
Annibale della Genga Name taken: Leo XII
← 1799–1800 1829 →

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Pius VII, who had been elected in 1800, had an unusually long pontificate, during which the Catholic Church had faced, in the French Revolution and its aftermath, a severe attack on its power and legitimacy. Pius himself had been a prisoner of Napoleon in France for six years.

During Pius VII's papacy, the cardinals had tended to divide into two groups, the zelanti and the politicani. The zelanti were more radically reactionary than the politicani and wanted a highly centralised Church and vehement opposition to the secularising reforms that had resulted in France. The politicani, though anti-liberal, were much more moderate and favoured a conciliatory approach to dealing with the problems that new ideologies and the incipient Industrial Revolution were creating. The leader of this faction was Pius VII's Cardinal Secretary of State, Ercole Consalvi, but the zelanti wanted a much less moderate pontiff and they set fervently to this task from the time of Pius VII's death.

The length of Pius VII's papacy had a significant influence because of the forty-nine electors who participated in the conclave, only Giulio Maria della Somaglia and Fabrizio Dionigio Ruffo were already cardinals when Pius VII was elected in 1800. Forty-seven of the forty-nine electors had had no experience electing a pope.^[1]

A number of cardinals were thought at the beginning of the conclave to be possible successors to Pius VII. Antonio Gabriele Severoli was at first seen as the most likely papabile, but the veto of Francis I, Emperor of Austria ruled him out when he looked to have a reasonable chance.

Francesco Saverio Castiglioni then emerged as the most likely candidate. The deceased pope Pius VII had in fact seemingly endorsed Castiglioni by having referred to the latter as Pius VIII,^[2][3] and indeed the candidate who was eventually elected predicted during the conclave that Castiglioni would someday reign under that name (as he did, succeeding the elected Leo in 1829). Castiglioni lost support in the 1823 conclave when the zelanti Cardinals came to realize that he was quite close to Consalvi.^[2][3]

Following the decline of Castigioni's support in this conclave, the experienced Cardinal della Somaglia then had a turn as a possible candidate, but for some the mere fact that he had signed his letters "Citizen Somaglia" during the occupation of the Papal States by Napoleon I of France ruled him out.

It was only then that the eventual winner, Vicar of Rome Annibale della Genga, who was being promoted by the zelanti, began to gather support. Whilst his tall, ascetic look and reactionary reputation was not an attraction for the politicani, the fact that he was seemingly at death's door seemed an attraction for those cardinals by now desperate for a resolution of the conclave. As a result, della Genga was elected on 28 September and took the regnal name Leo XII.

Leo XII was crowned as Pope on 5 October 1823.

• Fabrizio Turriozzi