Tu'an. The Partholon Series, #2

Five years after the events of Partholon, a juggernaut Red Army bears down on what's left of the poorly led, underequipped, and abandoned US 1st Combined Arms Division, nicknamed the Ghosts. Victory is assured; America will fall. But Sergeant Collier Rashkil and his lover, the beautiful and ruthless Major Rosa Arce, are Ghosts. And Ghosts don't die. Trapped in the swamplands of south Jersey, the Ghosts maintain a thin line of defense, their survival dependent on the intelligence Rosa and Collier provide.
But a reconnaissance mission goes very, very wrong, and Rosa discovers there is a traitor in their midst, one willing to sacrifice all their lives for a flawed Utopian vision. Based on an Irish legend, Tu'an is a hair-raising story of brutal combat, of betrayal, survival, and a love that no enemy, no war, not even death, can end.