After the Vulcans refusal to fight against the Romulans in the war they installed warp detection grids in all of the Coalition's systems. However, the Romulans discovered a way to circumvent these systems, allowing them to surprise unsuspecting Starfleet ships.
The USS Yeager was on patrol near Deneva when Romulan vessels entered the system and attacked the Yeager. The attack took Yeager entirely by surprise and led to it's destruction. Then, a short time later, the Tellarite warship Miracht entered the system and was engaged by the Romulans who attempted to take over the ship with their remote capture device. In order to stop the Romulans from capturing one of Tellar's strongest warships the ship's captain, Prev, fired a weapon into the ship's warp core, destroying the entire ship.
After the Romulans took out all space defenses, they landed troops on Deneva. The MACO contingent attempted to defend the planet, focusing on the capital city, but the Romulans' superior numbers overwhelmed and wiped out the defenders. Few people managed to escape the planet before it fell.
This was the first major loss for the United Earth, with a major colony world being lost. It also showed that the Romulans managed to find a tactic to elude the warp detection grids installed by the Vulcans. (ENT - The Romulan War novel: Beneath the Raptor's Wing)