What were the antecedent of the Greco-Persian Wars?

Greco-Persian Wars, also called Persian Wars, (492–449 BCE), a series of wars fought by Greek states and Persia over a period of almost half a century. The fighting was most intense during two invasions that Persia launched against mainland Greece between 490 and 479. Although the Persian empire was at the peak of its strength, the collective defense mounted by the Greeks overcame seemingly impossible odds and even succeeded in liberating Greek city-states on the fringe of Persia itself. The Greek triumph ensured the survival of Greek culture and political structures long after the demise of the Persian empire.

Explanation:

the answer to fill in the blank in this question would be the word resource, it is defined as the money, material or people (since human resource is a very valuable good) that a company or a person has at disposition (meaning they can use the resource as they see fit), to be able to complete a task or achieve a goal.