Which pope appealed to Charlemagne for help against the Lombards in 773?

The pope who appealed to Charlemagne for help against the Lombards in 773 was Pope Adrian I.

Adrian became pope in 772, succeeding Stephen III, as Lombard king Desiderius pressed into territories claimed by the papacy. The pope sought Frankish protection under the alliance established by Charlemagne’s father, Pepin the Short. When diplomacy failed, Adrian’s appeals helped prompt Charlemagne’s invasion of Lombard Italy.

Charlemagne crossed the Alps in 773, defeated Lombard forces and besieged their king at Pavia. The city fell in 774, Desiderius was deposed, and Charlemagne assumed the title King of the Lombards. The victory greatly expanded Frankish power in Italy and strengthened the papacy’s political position.

A common mix-up is Pope Leo III, who crowned Charlemagne emperor in 800. Stephen II, sometimes numbered Stephen III, belonged to the earlier generation and had worked with Pepin, while Adrian was the pope connected with the Lombard crisis of 773.