Through the Glass Darkly
The Klingon Cardassian Alliance in the Mirror Universe

Stardate 8391, Some 400 years have passed since the last crossover. The Terran-Vulcan Empire no longer exists. After numerous wars against the Klingons and Cardassians, which the Empire lose for the most part, the Terrans and Vulcans are defeated, subjugated and enslaved, as are the Tellarites, the Andorians, and other Alpha Quadrant species. Except for the Bajorans and Betazoids. The Bajorans are the administration and logistics arm of the Alliance and the Betazoids are used for interrogation and undercover work.   

The Klingon-Cardassian Alliance is now the most feared and formidable Alliance in the known galaxy. Their territory of space covers  most of the Alpha and Beta Quadrants, and is now busy conquering the Gamma Quadrant too. The Dominion are a formidable enemy due to their shapeshifting abilities, which allows for many new weapons to arise, such as a genetic disease that has been introduced into the great Link. The Dominion are dying but they struggle to fight on. The Jem Hadar are already enslaved by the Klingons and Cardassians as all Ketracel White is now manufactured by the K-C Alliance. The Vorta race were never genetically enhanced and so remained the primitive race they originally were. There is no Starfleet, but instead the Obsidian Order is the main military and intelligence arm of the Alliance.   

A new race also has arisen and is slowly becoming the majority within the K-C Alliance. That race is the mixed raced Klingons and Cardassians. Pure bred Klingons and Cardassians are slowly becoming the minority and dying out. There are also mixed race Betazoids with Klingons and Betazoids with Cardassians, via genetic fertilisation, in the hopes of breeding the telepathy gene throughout the Alliance.   

There are vague legends and stories amongst the Terrans of a time long ago when the Terran rebels obtained new technology from people in another universe, but since the Terrans were defeated there have been no further crossovers, and as such the stories are seen as legends that noone believes.   

Spock's one wish to be the person with a vision who changes history never came to pass. He disappeared into exile and died in Romulan space before they eventually moved to the Delta Quadrant.   

The Romulans, a group of renegade Vulcans who followed Surak into exile to practice his ideology of peace and logic, have quietly removed themselves from their former territory  wishing only to continue practising their peaceful and logical ways with no war. They apparently ended up in the Delta quadrant where they faced battles with the Vidiians, the Kazon, the Hirogen  and of course, the Borg.   

The Borg have assimilated most of the Delta quadrant but have not yet discovered the other quadrants (mainly because neither the Hansens nor the USS Voyager were ever in the Delta Quadrant). They are still bent on bringing order out of chaos, which is also  the Romulans ethic as well. The Romulans are therefore not assimilated but are charged with the task of finding other races which the Borg can assimilate and bring to order. It is an extraordinary but formidable new alliance. With their peaceful and logical way of life, and ancient legends of races back in the Alpha & Beta Quadrants, the Borg are slowly heading in that direction.