This is actually quite an unusual Greek play in that it does not deal with a mythological event. Granted Aristophanes deals with historical events, but he wrote comedy as opposed to tragedy (and I have explained elsewhere what is meant by Greek Tragedy). Excluding Aristophanes, The Persians is the only historical play that we have, and it is possible that it is the only historical play that was ever written during the classical period of Ancient Greece.
The play is about the Persian defeat at Salamis and is set entirely within the palace in Susa. Once again (as we always see) the unities of time and place are obeyed. While many seem to point to Aristotle as being the one who developed the unities, we must remember that Aristotle lived at least two generations after the great dramatists. Aristotle was the pupil of Plato who in turn was the pupil of Socrates, who was alive when Euripides and Sophocles were producing their plays.
This play is pretty much a pat on the back for the Athenians for winning what was considered to be the unwinnable war. It is also the second of the two sources that we have regarding the Battle of Salamis, however we need to remember that this was written from the Athenian viewpoint and in turn was written by Aeschylus' viewpoint, so it will automatically be biased in favour of the Athenians. However, it is a very useful source as numerous generals on the Persian side were named, and the play also outlines the Achameid Dynasty (the line of kings from whom Darius and Xerxes' were descended).
I won't go into too much detail regarding the battle of Salamis as this is discussed extensively in other places (by me as well as others). However the Battle of Salamis (which was a naval battle) is considered to be one of those points upon which of history swings. I am not entirely convinced by this argument, namely because I also believe in divine influence (as we can see from the Battle of Jerusalem when Sennacerib's army was completely destroy by something during the night) but then as we read through this play we can also see numerous references to the gods. However Aeschylus is theologically wrong when dealing with Persian religion. He seems to think that they had a polytheistic religion when in reality, by Xerxes' time, Persia had become Duotheistic, where two gods, equal and opposite, are forever slugging it out with each other (this is Xorastrianism in a really small nutshell).
One thing we must remember though is that Xerxes' survived. This is actually quite unusual for a king who is defeated in battle. Senacerib was killed by his sons upon his return to Ninevah, namely because his defeat was evidence that he no longer had the support of the gods. However, there are two possible answers to why he was no deposed. The first, and the more unlikely, is that Xorastrianism did not allow for this and that defeat is not necessarily the disapproval of the gods, but rather just bad luck. However, this, as far as I am concerned, is not a hugely satisfying answer.
The second answer to this question, I suspect, comes from the Bible, namely from the Book of Esther. Now the events in Esther occur during the reign of Xerxes (though there is debate as to whether it is Xerxes or not, however, for the purpose of my argument, I will take it as it stands) and deals with the festival of Purim. Here the Jews were marked for death, and it was only the intervention of Esther that enable them to be saved. Now, we ask the question of why were they marked for death, and what swayed Xerxes to listen to Haman (boo, hiss). It is clear from the book that Haman (boo, hiss) hated Mordechai (Yay) and the Jews, but I doubt he could have gone to Xerxes and said 'I hate these people, please wipe them out' (by the way, the 'yays' and the 'boo hisses' apparently come from the Jewish tradition when this book is read).
Okay, the Bible indicates that the events in Esther occurred in the twelfth year of the reign of Xerxes, which put it around 474 BC, where as the Persian Wars occurred in 480 to 479 BC, which is about 5 years afterwards. So when I think about it, it is unlikely the the attempted genocide of the Jews could have been related to the Persian Wars. The reason I suggested this is because it is common for a minority group to be blamed for an empire's failure, as we saw in Nazi Germany. So, I guess my thoughts about this pomgrom would be incorrect. However, let us further consider more evidence from the Bible. The feast at which Xerxes' first wife, Vashti, is set in the third year of his reign, which is before the Persian wars. However, it also appears that Esther was married to him probably a few months after, and was queen while Xerxes was away in Greece. This suggests that Amestris (the Greek name of Xerxes' wife) is in fact Esther. Now, I checked Wikipedia and they indicate that she was actually Vashti, but it then goes on to expound the Akkadian root of both words and this seems to indicate that Amestris is Esther as opposed to Vashti. I believe that that is the case, based on the biblical record (if it is correct that Ahasuerus and Xerxes are in fact the same person).
So, I guess my point is that the reason that Xerxes' was not deposed was because he was persuaded by Haman to blame the Jews for his defeat at Salamis, however through the intervention of Esther, this blame was then shifted back onto Haman, who was then subsequently executed. Anyway, this is all speculation, however I do enjoy speculating about ancient historical events, which is why I wrote this in the first place.