Katya's War — Jonathan L. Howard

The battle lines have been drawn. The people of Russalka turn upon one another in a ruthless and unwavering civil war even while their world sickens and the deep black ocean is stained red with their blood. As the young civilisation weakens, its vitality fuelling the opposing militaries at the cost of all else, the war drums beat louder and louder.
 
Katya Kuriakova knows it cannot last. Both sides are exhausted â€" it can only be a matter of days or weeks before they finally call a truce and negotiate. But the days and weeks pass, the death toll mounts, and still the enemy will not talk.  
 
Then a figure from the tainted past returns to make her an offer she cannot lightly refuse â€" a plan to stop the war. But to do it she will have to turn her back on everything she has believed in, everything she has ever fought for, to make sacrifices greater even than laying down her own life. To save Russalka, she must become its greatest enemy.