Old St. Peter's Basilica in the context of "Saint Peter's Basilica"

The Papal Basilica of Saint Peter in the Vatican (Italian: Basilica Papale di San Pietro in Vaticano), or simply St. Peter's Basilica (Latin: Basilica Sancti Petri; Italian: Basilica di San Pietro [baˈziːlika di sam ˈpjɛːtro]), is a church of the Italian High Renaissance located in Vatican City, an independent microstate enclaved within the city of Rome, Italy. It was initially planned in the 15th century by Pope Nicholas V and then Pope Julius II to replace the ageing Old St. Peter's Basilica, which was built in the fourth century by Roman emperor Constantine the Great. Construction of the present basilica began on 18 April 1506 and was completed on 18 November 1626.

Designed principally by Donato Bramante, Michelangelo, and Carlo Maderno, with piazza and fittings by Gian Lorenzo Bernini, Saint Peter's is one of the most renowned works of Italian Renaissance architecture and is the largest church in the world by interior measure. While it is neither the mother church of the Catholic Church nor the cathedral of the Diocese of Rome (these equivalent titles being held by the Archbasilica of Saint John Lateran in Rome), Saint Peter's is regarded as one of the holiest Catholic shrines. It has been described as "holding a unique position in the Christian world", and as "the greatest of all churches of Christendom".

The Donation of Pepin, or Donation of Pippin, was the transfer of Frankish territory in central Italy to Pope Stephen II made by Pepin the Short, King of the Franks, in 756. Credited with "effectively establishing the temporal authority of the papacy," the Donation took place amid the Byzantine Empire's decline in Italy and marked both "an important moment in the papal–Carolingian alliance" and an "important step" in the formation of the Papal States. With the fall of the imperial capital in Italy, Ravenna, to Aistulf, King of the Lombards in 751, "any semblance of imperial protection for Rome" had evaporated and the pope, who had technically been a Byzantine subject to this point, turned to Pepin for assistance. Pepin invaded Italy and, following an initial victory in 755, he decisively "defeated Aistulf and imposed a peace on him" in 756.

Pepin had promised Italian lands to Stephen II at a meeting in Quierzy, France in April 754. While this earlier promise is "often identified as the Donation," it may have been only verbal and "does not exist in written form." Papal accounts of the 754 promise state that Pepin "granted the pope the exarchate, including Ravenna, and the Roman duchy." In 756, the "Confession of St. Peter," a document listing the cities involved in the Donation, was "placed on the altar of Old St. Peter’s Basilica in Rome" along with "keys of the cities and territories in central Italy." The Confession document was created "following Pippin’s second invasion of Italy to assist the pope." After his initial invasion in 755, Pepin concluded the First Peace of Pavia, but Aistulf "refused to abide by" this agreement and attacked Rome. Pepin then defeated Aistulf again, imposing "heavy penalties" with the Second Peace of Pavia in 756.

Empress Matilda (c. 7 February 1102 – 10 September 1167), also known as Empress Maud, was Holy Roman Empress as the consort of Emperor Henry V from 1110 until his death in 1125, and was subsequently a claimant to the English throne during the civil war known as the Anarchy. Following the death of her father, King Henry I of England, she asserted her right to the English throne as his only surviving legitimate child and styled herself Lady of the English. However, her cousin Stephen of Blois was crowned king in her place.

Matilda was the only daughter and eldest child of Henry I, king of England and ruler of Normandy, by his first wife Matilda of Scotland, to survive to adulthood. As a child, she went to Germany where she was married to the future Henry V. She travelled with the emperor to Italy in 1116, was controversially crowned empress in St Peter's Basilica, and acted as the imperial regent in Italy. Matilda and Henry V had no children, and when he died in 1125, the imperial crown was claimed by his rival Lothair of Supplinburg.

The Leonine City (Latin: Civitas Leonina) is the part of the city of Rome which, during the Middle Ages, was enclosed with the Leonine Wall, built by order of Pope Leo IV in the 9th century.

This area was located on the opposite side of the Tiber from the seven hills of Rome, and had not been enclosed within the ancient city's Aurelian Walls, built between 271 and 275. After Christianity had risen to prominence and the Western Roman Empire had collapsed, the area had to be defended through the construction of a new wall, since it housed St. Peter's Basilica.