Though Charles Martel, being not king but only chancellor, never cared about titles, his son Pippin did, and finally asked the Pope
"who should be King, he who has the title, or he who has the power?"

The Pope, highly dependent on Frankish armies for his independence from Lombard and Byzantine power ( the Byzantine Emperor still considered himself to be the only legitimate "Roman Emperor", and thus, ruler of all of the provinces of the ancient empire, whether recognised or not ), declared for
"he who had the power" and immediately crowned Pippin.

Decades later, in 800, Pippin's son Charlemagne was crowned emperor by the Pope, further extending the principle by delegitimising the nominal authority of the Byzantine Emperor in the Italian peninsula, thus paving the way for the Frankish empire.