When Charlemagne's brother Carloman died in 771, his widow took her sons to Lombardy. The King of the Lombards attempted to get Pope Adrian I to anoint Carloman's sons as kings of the Franks. Resisting this pressure, Adrian turned to Charlemagne for help. Here he is depicted asking for aid from the king at a meeting near Rome.
Charlemagne did indeed help the pope, invading Lombardy, besieging the capital city of Pavia, and eventually defeating the Lombard king and claiming that title for himself.
Just for fun, try a jigsaw puzzle of this picture.