Lope de Vega chose to base his play Contra valor no hay desdicha upon sources by the historians Herodotus and Justin that recount the mythical events surrounding the birth and early life of King Cyrus the Great of Persia. Through his skilful selection and intricate blending of elements from both of these sources, Lope demonstrates his shrewd understanding of the construction of an entertaining play, the act of selection itself imbuing the dramatic elements of his own invention with a greater significance. This article sheds light on the multifaceted creative process involved in Lope's manipulation of historical sources for the creation of this one play. It does so through exploring those components taken by Lope from each historical source, those which he chose not to include, and those parts of the play that are his original invention, considering the import of each of these decisions in turn.