Calimshan is older than either of the other Empires of the Sands, first settled over 7,000 years ago by the Djen, a humanoid race from the Elemental Plane of Air. These Djen were known to be very magical, and during the course of their rule they developed many new spells previously not available in the Plane of Air.
The Djen prospered for over 1,000 years in Calimshan, but their reign was ended by an invasion of creatures and minions from the Plane of Fire. Some say this is where the bitter hatred between djinni and efreeti started, though others contend this was just a result of a hatred that was already there. Whatever the cause, the battle was long and bloody, and took over 100 years to complete. The Djen finally routed the attackers, but were greatly weakened in the attempt. They slowly declined, and the last mention of the Djen is just under 6,000 years old.
For the next 4,000 years Calimshan was dominated by nomadic tribes of humans. Tribes from various places - Chult, the Shaar, The Shining Plains, Chondath, even Amn and Cormyr - took turns dominating, only to be conquered by the next, nearly identical tribe.
Slowly, the nomadic nature of Calimshan began to change. As explorers and traders from Amn, Waterdeep, and Cormyr discovered the wonders of the area, some tribes began to settle down and develop new means of support, like fishing, farming, or trading. These communities began to band together for mutual protection, and soon a civilization was born. It was only 1,300 years ago that the Shoon Empire (now called Iltkazar) came into being.
The Shoons were a grand and glorious empire, and their excesses were the foundation of Calishite snobbery today. They grew wise and powerful in the ways of magic, and ships and caravans bearing the Shoon flag traveled across the Forgotten Realms. Shoon himself, a particularly powerful mage, created a book of great power during this time called the Tome of the Unicorn. The exact location of the Tome has been lost in time, but since the book is 2' by 3' and made of pure metal, it is likely to still be around... somewhere.
900 years ago the Shoon empire abruptly vanished. A great magical upheaval was suspected at first, but learned mages of other lands dispute the claim. A force that great, they say, would have disturbed magical powers and beings throughout the Realms, and that didn't happen. Sages who have studied the Shoon at great length have reached no definite conclusions, but the most popular theories today center around a plague or disease that decimated the population.
Today, the Shoon impact on Calimshan is still great. The grandeur of that empire is responsible, more than anything else, for the strong national character of Calimshan today. The ruins of the Shoon's greatest city, Monrativi Teshy Mir, can still be found in the wilderness to the west of the edge of the Forest of Mir (see below for more on Monrativi Teshy Mir).
Since the fall of Shoon, no force or people has risen to soley dominate the land. There are a half dozen or so major cities, each of which exerts its power over its own area. About 170 years ago, a man in Calimport amassed a large army and declared himself "Pasha" over the land. Before that army could march, however, the representatives of each major city met and agreed to recognize the Pasha's authority in limited areas, and to pay a small tribute to him; enough to pay for the works the Pasha was expected to do. The oldest son of each Pasha inherits the title; if there is no son, the mayors of each large city select a new one. The current Pasha, Rashid Djenispool, has ruled for over 18 years, and is the grandson of a pasha elected by the mayors of Calimshan 44 years ago.