What contributed to the transformation of the Frankish kingdom into an empire?

Even at the beginning of his reign, King of the Franks Charlemagne began the conquest of the peoples located nearby. Subjugating Saxony in 777 and conquering Bavaria in 787, under his command, he began to unite these peoples into one state.
Thanks to the further conquests of the Avars, he was able to unite almost the entire territory of Western Europe and Northern Italy.
Thus, a multinational state was formed with a huge army and the support of the Pope, who in 800 placed the Imperial crown on the head of Charlemagne.