What was the Donation of Constantine, and how did Lorenzo Valla expose the forgery?

The Donation of Constantine was a forged Roman imperial decree by which the emperor Constantine the Great supposedly handed authority over Rome and the western Roman Empire to the pope. It was composed probably in the 8th century, and it was used, especially in the 13th century, to support papal claims of political authority. For most of the Middle Ages almost everyone accepted it as genuine. It was exposed in 1439 and 1440 by Lorenzo Valla, an Italian Catholic priest and Renaissance humanist, with solid philological arguments.

The document was generous. "Constantine" professes Christianity and gives Pope Sylvester I imperial insignia and privileges, and the Lateran Palace. Rome, the rest of Italy and the western provinces of the empire are made over to the papacy. A story attached to it told how Sylvester rescued the Romans from a local dragon and miraculously cured the emperor's leprosy through baptism. It has been suggested that an early draft was made in the middle of the 8th century to help Pope Stephen II in his negotiations with Pepin the Short. In 1054, Pope Leo IX cited a large portion of it in a letter to the Patriarch of Constantinople, believing it genuine.

Valla attacked the text itself. Its language, he argued, pointed to the 8th century, not the 4th. It used the word "satrap", which he believed Romans such as Constantine would not have used. And the Latin was poor for such an important document, constantly switching tenses from "we have proclaimed" to "we decree". He also noted that the facts made no sense: Constantine could not legally have given the powers claimed, and there was no contemporary evidence of such a huge change.

It was not a safe thing to do. Valla worked at the court of Alfonso V of Aragon, who was in a territorial conflict with the Papal States, and in 1444 he was put on trial before the Catholic Inquisition. Alfonso protected him from imprisonment.