Britannica Sources used in this answer

Why did Pope Adrian I ask Charlemagne to invade Italy?

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Painting by Albrecht Durer of Charlemagne in ornate coronation robes and crown, holding a sword and orb, with gold embroidery and regal symbols behind him. — Charlemagne Charlemagne, oil on limewood by Albrecht Dürer, 1512; in the collection of the German National Museum, Nürnberg, Germany.

Pope Adrian I sought aid from Charlemagne because the Lombard king Desiderius had attacked papal possessions and was actively threatening the city of Rome.41

By the 8th century, the Byzantine Empire—which had historically provided protection to the papacy—was preoccupied with the threat of Islam and was unable to defend its Italian territories from the Lombards. Furthermore, the papacy had become increasingly estranged from the Byzantine emperors due to the Iconoclastic Controversy.23

Facing these threats and the failure of the Byzantine Empire to provide adequate protection, Pope Adrian I invoked the established alliance between the Papacy and the Frankish monarchy. Charlemagne responded by invading Italy in 773, defeating the Lombards, and capturing Desiderius in 774, which effectively ended the Lombard kingdom.41