One of the main ancient sources of knowledge with respect to the Persian Empire is the book The Persian Wars, which was written by Herodotus, who is considered one of the first historians of the world. Despite his fame, the fact that he was Greek, implies that his narration of events might have been biased against the Persians, as, in the ancient world, Greeks and Persians were enemies: thus the title of the book, which refers to the battles the Greek states held with Persia over the course of almost fifty years, in the first half of the 5th century BC. The Greeks and the Persians were very different in many ways. While the Greeks, to give an example, highly endorsed slavery, which was widespread throughout all the city-states, Zoroastrianism -- which had become the Persian Empire’s official religion -- prohibited it. In addition, Zoroastrianism was a monotheistic religion, meaning that it only comprised one god, whereas the Greek believed in a multitude of deities, such as Zeus (god of the sky), Hera (goddess of marriage), Poseidon (god of the sea), Demeter (goddess of agriculture), Ares (god of war), and Athena (goddess of wisdom), among many more. In any case, the main root of the wars between both peoples was the desire of the Persians to expand their domains even further, eventually clashing with Greek city-states, which were actually independent and did not regard themselves as belonging to a single “Greece” until the wars developed.