Pope Sixtus IV (July 21, 1414 – August 12, 1484), born Francesco della Rovere, was Pope from 1471 to 1484. He founded the Sistine Chapel where the team of artists he brought together introduced the Early Renaissance to Rome with the first masterpiece of the city's new artistic age.
With his election to pope, Sixtus IV declared a renewed crusade against the Ottoman Turks in Smyrna. Fund-raising for the crusade was more successful than the half-hearted attempts to storm Smyrna, with little to show in return. Some fruitless attempts were made in unification with the Greek Church. For the remainder of his pontificate he turned to temporal issues and dynastic considerations. Sixtus continued the dispute with Louis XI of France (1461–1483), who upheld the Pragmatic Sanction of Bourges (1438), according to which papal decrees needed royal assent before they could be promulgated in France. This was a cornerstone of the privileges claimed for the Gallican Church and could never be shifted as long as Louis XI maneuvered to replace Ferdinand I of Naples with a French prince, thus being in conflict with the papacy, which as a princely strategist could not permit it.
Like a number of Popes, Sixtus IV adhered to the system of nepotism. In the fresco by Melozzo da Forlì he is accompanied by his Della Rovere and Riario nephews, not all of whom were made cardinals: the protonotary apostolic Raffaele Riario (on his right), the future Pope Julius II (1503–1513) standing before him, and Girolamo Riario and Giovanni della Rovere behind the kneeling Platina, author of the first humanist history of the Popes. His nephew Pietro Riario also benefited of his nepotism, becoming one of the richest men in Rome and being entrusted of Sixtus IV's foreign policy, but died prematurely in 1474, his role passing to Giuliano della Rovere.
The secular fortunes of the Della Rovere began when Sixtus invested his nephew Giovanni with the signoria of Senigallia and arranged his marriage to the daughter of Federico III da Montefeltro, duke of Urbino; from the union came a line of Della Rovere dukes of Urbino that lasted until the line expired, in 1631.
In his territorial aggrandizement of the Papal States Sixtus IV's niece's son Cardinal Raffaele Riario, for whom the Palazzo della Cancelleria was constructed, was a leader in the 1478 failed "Pazzi conspiracy" to assassinate both Lorenzo de' Medici and his brother and replace them in Florence with Sixtus IV's other nephew, Girolamo Riario. Francesco Salviati, archbishop of Pisa and a main organizer of the plot, was hanged on the walls of the Florentine Palazzo della Signoria. To this Sixtus IV replied with an interdict and two years' of war with Florence. He also encouraged the Venetians to attack Ferrara, which he wished to obtain for another nephew. The angered Italian princes allied to force Sixtus IV to make peace, to his great annoyance.
As a temporal prince who constructed stout fortresses in the Papal States, Sixtus IV committed himself to Venice's aggression against Ercole I d'Este, Duke of Ferrara, inciting the Venetians to attack in 1482 in the so-called War of Ferrara. Their combined assault was opposed by an alliance of the Sforzas of Milan, the Medicis of Florence along with the King of Naples, normally a hereditary ally and champion of the Papacy. For refusing to desist from the very hostilities that he himself had instigated (and for being a dangerous rival to Della Rovere dynastic ambitions in the Marche), Sixtus IV placed Venice under interdict in 1483.
Sixtus IV consented to the Spanish Inquisition and issued a bull in 1478 that established an Inquisitor in Seville, under political pressure from Ferdinand of Aragon, who threatened to withhold military support from his kingdom of Sicily. Nevertheless, Sixtus IV quarrelled over protocol and prerogatives of jurisdiction, was unhappy with the excesses of the Inquisition and took measures to condemn the most flagrant abuses in 1482. In ecclesiastical affairs, Sixtus IV instituted the feast (December 8) of the Immaculate Conception of the Virgin Mary. He formally annulled (1478) the confusedly reformist decrees of the Council of Constance.
Sixtus IV is one of several Popes suspected of being homosexual. The basis of this being the diary records of Stefano Infessura (1440-1500) who recorded documented episodes and unsubstantiated rumours. This included accusations that Sixtus had awarded benefices and bishoprics in return for sexual favours. It is clear though that Sixtus rather favoured his relatives instead in order to have faithful executors of policy. However an exception was Giovanni Sclafenato, who was created a cardinal according to the papal epitaph on his tomb for "ingenuousness, loyalty and his others gifts of soul and body". Later accusations of sodomy by Protestants can be dismissed as propaganda.
As a civic patron in Rome, even the anti-papal chronicler Stefano Infessura agreed that Sixtus IV should be admired. The dedicatory inscription in the fresco by Melozzo da Forlì in the Vatican Palace records: "You gave your city temples, streets, squares, fortifications, bridges and restored the Acqua Vergine as far as the Trevi..." In addition to restoring the aqueduct that provided Rome an alternative to the river water that had made the city famously unhealthy, he restored or rebuilt over 30 of Rome's dilapidated churches, among them San Vitale (1475) and Santa Maria del Popolo, and added seven new ones. The Sistine Chapel was sponsored by Sixtus IV, as was the Ponte Sisto, the Sistine Bridge – the first new bridge across the Tiber since antiquity – and the building of Via Sistina (later named Borgo Sant'Angelo), a road leading from Castel Sant'Angelo to Saint Peter. All this was done to facilitate the integration of the Vatican Hill and Borgo with the heart of old Rome. This was part of a broader scheme of urbanization carried out under Sixtus IV, who swept the long-established markets from the Campidoglio in 1477 and decreed in a bull of 1480 the widening of streets and the first post-Roman paving, the removal of porticoes and other post-classical impediments to free public passage.
At the beginning of his papacy in 1471, Sixtus IV donated several historically important Roman sculptures that founded a papal collection of art that would eventually develop into the collections of the Capitoline Museums. He also refounded, enriched and enlarged the Vatican Library. He had Regiomontanus attempt the first sanctioned reorganization of the Julian calendar and increased the size and prestige of the papal chapel choir, bringing singers and some prominent composers (Gaspar van Weerbeke, Marbrianus de Orto, and Bertrandus Vaqueras) to Rome from the North.
His bronze funerary monument, now in the basement Treasury of St. Peter's Basilica, like a giant casket of goldsmith's work, is by Antonio Pollaiuolo. The top of the casket is a lifelike depiction of the pope lying in state. Around the sides are bas relief panels, depicting with allegorical female figures the arts and sciences (Grammar, Rhetoric, Arithmetic, Geometry, Music, Painting, Astronomy, Philosophy, and Theology). Each figure incorporates the oak tree ("rovere" in Italian) symbol of Sixtus IV. The overall program of these panels, their beauty, complex symbolism, classical references, and arrangement relative to each other is one of the most compelling and comprehesive illustrations of the Renaissance worldview.
In addition to being a patron of the arts, Sixtus IV was a patron of the sciences. Before becoming Pope, spent time at the then very liberal and cosmopolitan University of Padua, which maintained considerable independence from the Church and had a very international character. As pope, he issued a papal bull allowing local bishops to give the bodies of executed criminals and unidentified corpses to physicians and artists for dissection. It was this access to corpses which allowed the anatomist Vesalius along with Titian's pupil Jan Stephen van Calcar to complete the revolutionary medical/anatomical text De humani corporis fabrica.