Mesopotamia was known as the Fertile Crescent because

También llamado "media luna fértil", es una región histórica que se corresponde con parte de los territorios del Levante mediterráneo, Mesopotamia y Persia. ... El término fue acuñado por el arqueólogo James Henry Breasted (Universidad de Chicago) por la forma de Luna creciente del área geográfica referida.

Traduccion ingles:

Also called "fertile crescent", it is a historical region that corresponds to part of the territories of the Mediterranean Levant, Mesopotamia and Persia. ... The term was coined by archaeologist James Henry Breasted (University of Chicago) for the crescent moon shape of the referred geographic area.

Explanation:

president thomas jefferson commissioned the expedition shortly after the louisiana purchase in 1803 to explore and to map the newly acquired territory, to find a practical route across the western half of the continent, and to establish an american presence in this territory before britain and other european powers