Xerxes

Xerxes (クセルクセス kuserukusesu) (spelled 'Cselkcess' in Viz manga) was an ancient country that existed in the Great Desert to the east of Amestris four hundred years before the start of the series. Despite having had a remarkably advanced society for their era, the entire population of Xerxes was mysteriously wiped out in a single night, leaving only ruins behind.

History
Xerxes was a relatively peaceful, thriving land despite the harsh conditions of the desert that surrounded it and, at the time of its sudden disappearance, had a population of over a million people. Xerxes appears to have had a markedly intellectual culture that fostered written language as well as philosophical and alchemical studies. Though slavery seems to have been a common practice, it was possible for slaves to rise out of thralldom through education, even going so far as to become members of high society. Xerxes was also a centralized monarchy, with a King and royal court governing from the capital, a castle city in the center of the country. For the most part, Xerxesian people are characterized by having light complexions as well as gold-colored hair and eyes; however, there does seem to have been a minority of citizens with darker skin tones and similarly light-colored hair.

In the current continuity, the ruins of Xerxes consist mainly of the decaying remnants of the king's castle and capital city; the famous, yet remote site acts as an oasis for travelers venturing across the Great Desert from Amestris to Xing or vice-versa. Additionally, the ruins serve as a haven in which a number of Ishvalan refugees have taken refuge in the years after the Ishval Civil War.

Destruction
When a prominent alchemist within the King's court created a Homunculus with extensive arcane knowledge of alchemy, the aging monarch saw his chance to transcend his dreaded and inevitable death through immortality. The creature agreed to become the royal court's primary consul on the subject of alchemy-based immortality and persuaded the desperate king to construct an array that would encompass all of Xerxes and use all the people's souls to create a source of immortality. A large trench was dug according to the Homunculus' specifications, while the citizens were given the impression that their dutiful king had ordered the construction of irrigation canals to defend their crops against drought. Beginning with the northern town of Bonath, five towns around the circle were destroyed - the homes burned down and the people and livestock slaughtered - carving a crest of blood into the land. However, when it came time to activate the circle, it became immediately apparent that the Homunculus had deceived the court. By fashioning the array around himself and his caretaker instead of around the King, as he had claimed to have done, the Homunculus was able to cleverly steal the lives of Xerxes' citizens, royalty and his creator for himself and use them to create a pair of immortal bodies, each housing half of the nation's souls. Xerxes' entire population was wiped out instantly, leaving only the two immortals to survive as Human Philosopher's Stones. The two parted ways, leaving the dead country of Xerxes behind. However, the Homunculus would return to the site of his sin centuries later to destroy the evidence of what he had done; he had his son Gluttony devour some of the Transmutation Circle that had been inside the castle so as to keep others from realizing the truth behind the fall of Xerxes and the fact that the tragedy was to be repeated.

The 2003 Series
The nation of Xerxes is only mentioned briefly in episode 27, when Al and Ed study alchemy and talk about the risk in human transmutation and Al mentions "the country they say died in a single night". Ed is convinced that this story is just a fairytale, but refers to it as "The story of the Philosopher's Stone in the eastern desert", meaning they talk about Xerxes and not the Underground City beneath Central.

Due to the split between the manga and 2003 anime plot lines, the Underground City appears to have been a sort of parallel to Xerxes in the 2003 anime series, as its ancient civilization had been wiped out to create a Philosopher's Stone by Hohenheim of Light and Dante four hundred years before the start of the series and had been dragged underground so as to keep others from discovering the truth.

The Video Game Series
Siam-Sid is the former capital of the country of Lebis in the video game Fullmetal Alchemist 2: Curse of the Crimson Elixir. The former king of Lebis had attempted to create a perfect human being and the ritual involved therein, wiped out the entire civilization in a single night.

Lebis is most likely a misspelling of Rebis, an important concept in medieval alchemy, due to the translation from English to Japanese and then back again.

Trivia

 * Judging from the architecture, style of dress and cultural similarities, it is more than likely that Xerxes is based on the civilization of Persian Empire. This would also tie into the fact that much of the Fullmetal Alchemist canon is based on Ancient Greek culture and hermeneutics. Xerxes could also be based on the lost civilization of Atlantis, since both were advanced civilizations and disappeared in a single night.
 * Xerxes was the name of the Emperor who ruled the Persian Empire from 485 to 465 BC. The country was named as a little Easter egg from Arakawa, who also named the other central country of the series, Amestris, after his wife.
 * As direct descendants of Van Hohenheim, the last living Xerxesian, both Edward Elric and Alphonse Elric are half Xerxesian on their father's side. Both brothers even share the features of Xerxesian people; golden hair, eyes and tan skin.
 * Due to the lack of an "x" sound in the Japanese language, the name of the country is pronounced and written "Ku-se-ru-ku-se-su" in Japanese, prompting the publishers of the manga's English translation to mis-transliterate the word as "Cselkcess". Though original Japanese fact books spell the name similarly, the 2009 anime series has rectified this mistake, displaying the word as "Xerxes". As such, the English dub reflects the proper pronunciation based on the proper spelling. Incidentally, the proper English pronunciation of the word begins with a "z" sound.
 * Curiously, the Nationwide Transmutation Circle at the heart of Xerxes appears in the begining of the first opening of 2009 series.
 * Xerxes had a total population of 1,072,658 people.