A secret attack on the Founders' homeworld by the Tal Shiar and the Obsidian Order, which ended in a Jem'Hadar ambush that destroyed the joint fleet.
The Battle of the Omarion Nebula was a pre-emptive strike against the Dominion carried out in 2371 by the Romulan Tal Shiar and the Cardassian Obsidian Order. The two intelligence services secretly assembled a fleet of cloaked ships and sent it through the Bajoran wormhole to destroy the Founders' homeworld in the Omarion Nebula, hoping to eliminate the Dominion threat in a single blow.
The episodes Improbable Cause and The Die Is Cast follow the plan through the eyes of Elim Garak, who is offered a return from exile by his former mentor Enabran Tain, and Odo, who is taken prisoner aboard the flagship. Tain orders Garak to interrogate Odo about the Founders, and Garak tortures him before freeing him. When the fleet arrives, the planet is empty. The Romulan officer Lovok is revealed to be a Changeling, and a large Jem'Hadar force ambushes and destroys the fleet.
The defeat effectively destroyed the Obsidian Order and badly weakened the Tal Shiar, shifting the balance of power in the Alpha Quadrant ahead of the Dominion War.