In Aeschylus's Persians, what historical event does the book represent? Who are the two major opposing factions in the book? Why are the Persians called barbarians? What kinds of images are conveyed by the stories of death and despair told by the messenger? Do you think the author is siding with the Greeks or the other side? What do you think is Aeschylus’s message?

The Persians by Aeschylus is a radically unconventional Greek tragedy in several ways. First, most Greek tragedies are set in a semi-mythical heroic age, around the time of the Trojan war. Unlike these, The Persians is a contemporary drama, portraying the war the Greeks recently won against the Persians. Even more interestingly, it tells the story from the point of view of a Persian queen, Atossa, rather than the Greek point of view. 
The two major factions in the book are the Greeks and the Persians, although the Greeks are only described as opponents in war; they do not appear directly in the play.
The term "barbarian" is an anglicized version of a Greek work used to refer to foreigners who do not speak Greek. Although the Greeks considered non-Greeks inferior, in actual fact, the Persian Empire was a wealthy, powerful, and sophisticated society.
The messenger's description of the defeat of the Greeks evokes a level of religious as well as personal horror for the Greek audience because it describes corpses left unburied and without funeral rites, something that condemns the souls to a dismal afterlife. 
In terms of sides, Aeschylus is a patriotic Greek and the ending of the play has the wise Darius favor the Greeks. On the other hand, the play does not condemn Persia or show all the Persians as uncivilized or immoral; Xerxes (like many other protagonists of Greek tragedy) is portrayed as someone who errs and brings disaster through arrogance. Aeschylus's portrait of the war (as well as his other works which portray wars) suggests that he sees wars as inherently tragic and engendering much suffering, especially among the innocent such as the elderly, women, and children. Thus one could say that the message is one about the horrors of war.