Due to interference from secular rulers and the cardinals' disregard for their supposed isolation from the outside world, it was the longest conclave of the 16th century.

[1] His church reforms had mainly been based on repressive measures such as the Inquisition and the Index of Forbidden Books – he had no confidence in the Council of Trent, dissolving it in 1552 and not reviving it.

Philip II of Spain preferred cardinals Carpi, Morone, Puteo, Medici and D'Oler – in short, any candidate other than d'Este or a Frenchman.

For Carlo Carafa, choosing the new pope was literally a matter of life and death and so he mainly used the conclave to obtain guarantees that he and his relatives would not be punished for their abuses.

The papacy was under criticism for failing to address abuses, and the college of cardinals was split between moderates and conservatives, as well as along national lines.

On 22 September the French tried to get cardinal Tournon selected, but his chances were dashed by Carafa's opposition, who supported the Spaniard Pacheco.

A few days later, the French agreed with Sforza, leader of the Spanish faction, to support cardinal Gonzaga and push through his election by acclamation.

On 25 September Philip II's ambassador Vargas arrived in Rome and under his auspices Sforza, Farnese and Carafa met on 2 October.

Francis II of France and Ferdinand I, Holy Roman Emperor also sent letters to cardinals recommending Gonzaga's candidacy.

This breach of the canonical rule that the conclave be held in secret and without any influence from secular leaders outraged the people of Rome into protest, but du Bellay (dean of the College of Cardinals) rejected the objections.

In the second half of October Carafa broke his alliance with Sforza, as Philip II decided to return the fiefdom Palli Colonnie Marcantonio and ordered the Spanish cardinals to prevent the selection of Gonzaga at all costs.

However, in the meantime, Carafa received a letter removing the expected guarantees from Philip and he and the French returned to their alliance with the Spanish party.

The protracted conclave led to increasing concern on the streets of Rome, especially since the camerlengo was forced to reduce troop numbers due to financial problems.

The French were eventually persuaded to back cardinal Medici, who was also strongly supported by the Duke of Florence and Vice-Chancellor Alessandro Farnese.

On the evening of 24 December, 44 cardinals gathered in the Sistine Chapel and elected Giovanni Angelo Medici as pope by acclamation, ending the longest conclave of the 16th century.

The next morning, on 25 December, a Scrutiny was held and forty-four ballots were cast; two cardinals were absent, Saraceni and du Bellay.

Giovanni de' Medici took the name Pius IV and on the feast of the Epiphany on 6 January 1560 the Cardinal protodeacon Alessandro Farnese crowned him with the papal tiara.