Stephen II is the pope under whom the medieval Papal States were born and the foundational alliance of the papacy with the Carolingian Frankish kingdom was forged — making his pontificate the single most consequential turning-point in the political history of the Western Catholic Church.
Elected on March 26, 752, after the four-day pontificate of an earlier Stephen who died before consecration (Stephen II/III numbering is disputed), he immediately faced the Lombard king Aistulf, who had seized the Exarchate of Ravenna in 751 and was demanding the surrender of Rome itself.
Failing to obtain Byzantine help, Stephen made the unprecedented decision in 753 to cross the Alps in winter and appeal personally to the Frankish king Pippin the Short — the first reigning pope ever to travel to the Frankish kingdom. At the royal villa of Ponthion in January 754, Pippin prostrated himself before the pope and acted as his strator (groom), leading the pope's mule on foot for a stretch.
At the Synod of Quierzy (April 754), Pippin issued the foundational 'Donation of Pippin' — promising to wrest the conquered exarchate from Aistulf and grant it not to the Eastern Emperor but to the Apostolic See of Peter. Stephen, in turn, re-anointed Pippin and his sons Charles (Charlemagne) and Carloman as Kings of the Franks at Saint-Denis on July 28, 754 — establishing the foundational ritual of papal coronation of Western emperors that would shape European political theory for the next thousand years.
Pippin invaded Italy in 755 and 756, defeated Aistulf, and at the Treaty of Pavia (June 756) compelled him to surrender the keys of the Exarchate cities — physically placed by Pippin's representatives on the tomb of St. Peter in Rome. This act constituted the foundational legal title-deed of the Papal States, which would survive in continuous papal sovereignty until the Italian unification of 1870 (a 1,114-year temporal sovereignty). Stephen died on April 26, 757.