Around 500 BC, following the Achaemenid (Persian) conquest of Mesopotamia under Darius I, Aramaic (as had been used in that region) was adopted by the conquerors as the "vehicle for written communication between the different regions of the vast empire with its different peoples and languages. The use of a single official language, which modern scholarship has dubbed Official Aramaic or Imperial Aramaic, can be assumed to have greatly contributed to the astonishing success of the Achaemenids in holding their far-flung empire together for as long as they did". In 1955, Richard Frye questioned the classification of Imperial Aramaic as an "official language", noting that no surviving edict expressly and unambiguously accorded that status to any particular language. Frye reclassifies Imperial Aramaic as the lingua franca of the Achaemenid territories, suggesting then that the Achaemenid-era use of Aramaic was more pervasive than generally thought.