Battle of the Omarion Nebula

The Battle of the Omarion Nebula was the first major battle between the forces from the Alpha Quadrant and the Dominion. A fleet of twenty starships from the Cardassian Obsidian Order and the Romulan Tal Shiar attempted to launch a pre-emptive strike against the Dominion, attacking the Founder's homeworld in the Omarion Nebula.

The joint fleet was commanded by Enabran Tain and Colonel Lovok entered the Gamma Quadrant through the Bajoran wormhole and travelled cloaked to the Founder's homeworld hoping to catch the leaders of the Dominion unaware. The fleet then decloaked and began an orbital bombardment of the planet's surface. The fleet soon discovered that the Founders were no longer on the planet and 150 Jem'Hadar warships appeared out of the nebula.

It was then revealed that Lovok had been replaced by a Changeling and that the Dominion had known about the attack long before it was launched. The battle was swift and brutal, the Jem'Hadar greatly outnumbered the joint fleet and destroyed it, killing most of the personnel. Several, such as Enabran Tain, were captured and interned. 

In the aftermath of the battle the Obsidian Order was fatally crippled. Its fall would lead to a civilian uprising that saw the Detapa Council gain control of the Cardassian Union. This event and other activities by Changeling infiltrators would eventually lead to the Cardassian Union joining the Dominion, giving them a foothold in the Alpha Quadrant and lead to the Dominion War. 

The Tal Shiar was severely weakened as well, its failed attack lost it influence on Romulus, also weakening the positions of its allies and supporters in other sectors of the Romulan Star Empire.