Why were the names Persia and Iran both used for the Iranian plateau?

The names Persia and Iran have both been used to describe the Iranian plateau due to a distinction between internal self-identification and external naming conventions.

The name Iran, meaning "Land of the Aryans," has been used traditionally by the people of the region to refer to their own country. Conversely, the term Persia originated from a specific southern region known as Persis (or Pārs/Parsa, modern Fārs).12

When the Achaemenid Empire—which was native to Persis—expanded its political influence, ancient Greeks encountered these inhabitants and began applying the name "Persia" to the entire Iranian plateau. This external usage was further reinforced by the later Sasanian dynasty, which was also from Persis and dominated the region until the 7th century CE.2

While the terms are often used interchangeably for historical contexts, the government of Iran officially requested the international use of the name "Iran" in 1935 to replace the external designation of "Persia".12