The Third Battle of Deneva occured in 2159, during the Earth-Romulan War. Romulan forces attacked and conquered the Human colony world of Deneva for a second time after its liberation three years earlier.
After some successes in the early years of the war, Earth's war fortunes started to turn, and in the last months of 2159, they suffered a series of defeats, including the loss of Deneva and its crucial mineral resources.
However, contrary to the massacre of the planets colonists in 2156, this time around most of them were evacuated from the system or retreated into mines in the asteroid ring. From there they waged a guerrilla style campaign by redirecting smaller asteroids to collide with the planet, thereby keeping the Romulans from consolidating their positions. (ENT novels: Beneath the Raptor's Wing, A Choice of Futures)