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The Battle of the Omarion Nebula was the first major armed engagement between forces of the Alpha and Beta Quadrants and the Dominion, as well as the only large-scale engagement to take place in the Gamma Quadrant. In the battle, a combined Obsidian Order and Tal Shiar fleet attempted to launch a preemptive strike on the Founders' homeworld in the Omarion Nebula, only to be ambushed and completely destroyed by the Jem'Hadar.
By any measure the Battle of the Omarion Nebula was a catastrophe for the Romulan and Cardassian forces resulting in destruction not seen since the Battle of Wolf 359; the Tal Shiar suffered a severe setback from which it would not recover from for some time, and the Obsidian Order ceased to exist altogether. The Founders had identified both organizations as a definite threat due to their efficient and ruthless natures, and had exploited Tain's plan in order to wipe out both of them in a single stroke. Their victory had been so overwhelming that the Dominion had essentially eliminated two of the major powers in the Alpha and Beta Quadrants from challenging them in the short-term; after their losses neither the Cardassians nor the Romulans would be very willing to wage war against the Dominion.