John XIII (Latin: Ioannes XIII) (1032 - 1090) was King of Italy and Pope of the Catholic Church from 1081 to 1090.
John was born under the name Theobald to an Italian noble family of Frankish origin. As a younger son of his family, he was encouraged to pursue an ecclesiastical career, becoming Bishop of Verona in 1058 before being elected to the papacy in 1081.
The Papal Kingdom, which controlled most of northern and central Italy, had always had uneasy relations with the Roman Empire which controlled parts of the south, and raiders regularly crossed the border in both directions. John, however, actively encouraged the raids and provoked much anger when he openly paraded Roman slaves before the Roman ambassador to his court. In 1082, therefore, the Roman governor in Naples issued an ultimatum to John, demanding that he cease the raids, release all captives and pay reparations. When John refused, the governor sought and received permission from Emperor Alexios I to invade Italy and depose John.
John's army was not strong enough to resist even the modest provincial forces of the governor, and he was forced to abandon all of Suburbicarian Italy. He escaped capture however and was able to set up a new court in Milan, where he focused his efforts on issuing propaganda designed to garner Western European support for a retaliatory expedition. He did not live long enough to see these plans accomplished, however - instead, it was left to his successor, Urban II, to begin the Great Crusade.