At Adrian's urging, Charlemagne crossed the Alps and defeated the Lombard king, Desiderius, who had annexed papal territory; this marked the end of the Lombard kingdom.
In 774, during the siege of Pavia, Charlemagne went to Rome and confirmed and expanded the donation of the Papal States by Pepin.
Adrian then acknowledged Charlemagne's protectorate over Italy, effectively founding the temporal power of the popes.
Adrian supported Empress Irene in her struggle against iconoclasm.
Sent legates to the Second Council of Nicaea.
Condemned the heresy of Adoptionism.