In 2156, Praetor D'deridex ordered Admiral Valdore to attack Haakona because D'deridex's father, also a Praetor, was unsuccessful in conquering the planet before. Valdore thought that the war was a foolish distraction from the Earth-Romulan War, but he carried out the Praetor's orders anyway.
The initial assault on Haakona was quickly repulsed by the Haakonans using a previously unknown weapon and then the Haakonans began attacking Romulan outposts. (ENT - The Romulan War novel: Beneath the Raptor's Wing)
Spoiler warning: Plot and/or ending details follow.
In 2157, Romulan Admiral Dagarth ended the war by deploying a mutagenic virus against Haakona Prime. This virus changed the entire Haakonan race into a different one. Admiral Valdore was extremely mad at Dagarth for circumventing his orders and had Dagarth executed. (ENT - The Romulan War novel: To Brave the Storm)