Long ago, the Changelings were a race of peaceful explorers, travelling through the universe to extend their knowledge of it. But they were met with hostility and persecution. Therefore they became isolationists and formed the Dominion - a vast alliance of worlds ruled with an iron fist by the Changelings, who rarely reveal themselves, and are thought of as Gods by inhabitants of the Gamma Quadrant.

The Changelings genetically created a breed of aliens known as the Jem'Hadar - a ruthless and deadly efficient army of soldiers. The Jem'Hadar are addicted to a drug that only the Changelings can provide, thus forcing their allegiance to the Dominion.

Throughout the Gamma Quadrant, the Jem'Hadar are known for destroying cultures which will not join the Dominion. Therefore most planets will join without question. But now that access to the Alpha Quadrant has been established, the Changelings have taken up a different way of sustaining order...

By infiltrating the governments of the major (and minor?) powers of the Quadrant, the Changelings look to destabilise the shaky peace treaties established; weakening the Quadrant for an easy take-over by the Dominion.

A tactic they seem to have adopted with the Cardassians is weaken them so far that eventually they will join the Dominion... Firstly, the Changelings cause the Obsidian Order and the Romulan Tal Shiar through the Wormhole for a first strike against the Dominion. But when the fleet arrives at the Changeling Homeworld, they are ambushed by hundreds of JemHadar ships, which eventually destroy the Cardassian-Romulan fleet. This weakens the Cardassians and Romulans substantially, even causing the Romulans into a period of isolation.

The second ploy to weaken Cardassia by the Changelings, was to make the Klingons believe that Cardassia had been infiltrated, causing the Klingons to invade Cardassian space, destroy half their fleet and capture dozens of Cardassian worlds.  This move by the Dominion also broke the Federation-Klingon peace treaty, causing even more pandemonium in the Quadrant.  By now the Cardassians were not the powerful race that they once were, half their fleet destroyed and most of their worlds annexed, and Gul Dukat saw no other course of action than to open negotiations with the Dominion - creating an alliance between the Dominion and Cardassia.

This proved to be a wise move for the Changelings - they can now use the Cardassians to defeat the minor Alpha Quadrant races, then destroy the Cardassians when they're finished with them.  But by increasing the Dominion threat on the Alpha Quadrant, the Changelings caused the Federation, Klingons, and Romulans to re-establish their shaky treaty, and the Cardassians/Dominion turned to a new ally - the Breen.  After a long and costly war for both sides the combined UFP/Klingon/Romulan alliance defeated the Dominion and restored peace to the Alpha and Beta Quadrants.

For more information go to The Dominion Guide

Changelings are a race of shapechangers from the Gamma Quadrant. They can appear as virtually anything they wish.  They must return to their natural form, a gel-like liquid, once every 16 hours.  Without this rest, they will find it increasingly harder to maintain the morphed shape.  They only need to stay in this "natural" form for one hour. 

Their mimicry abilities give them an uncanny ability to duplicate virtually anyone (or anything) they have ever seen.  They can change form, shape, size, and color at will, and can do so in just a few seconds.  Combine the above with their Body of Plasm, and they are a very effective double agent, among other things. You would have to know the victim of a Changeling switch -very- well in order to detect their impersonation at all. 

The races of the Alpha Quadrant have recently discovered that pieces removed from a Changeling's body will revert to their natural gel-like form. Blood tests are now standard protocol throughout the Federation and Klingon Empire to weed out Changeling infiltrators.  Members of this race can stretch their bodies even while maintaining a regular form. They do not need to eat or drink, at least not in any known manner, and their need of sleep is quite different from normal humanoids. They do not even need to breathe, but their bodies cannot survive in the vacuum of space without a suit of some type. 

Changelings have a fanatical hatred of  "solids" and have attempted to infiltrate militaries and governments of several Alpha Quadrant races, including the Federation.  It is not known how far this process has gone.