Guns of the dawn - Adrian Tchaikovsky

Denland and Lascanne have been allies for generations, but now the Denlanders have assassinated their king, overthrown the monarchy and marched on their northern neighbour. At the border, the war rages; Lascanne's brave redcoats against the revolutionaries of Denland.

Emily Marshwic has watched the war take her brother-in-law and now her young brother. Then comes the call for more soldiers, to a land already drained of husbands, fathers and sons. Every household must give up one woman to the army and Emily has no choice but to join the ranks of young women marching to the front.

In the midst of warfare, with just enough training to hold a musket, Emily comes face to face with the reality: the senseless slaughter; the weary cynicism of the Survivor's Club; the swamp's own natives hiding from the conflict.

As the war worsens, and Emily begins to have doubts about the justice of Lascanne's cause, she finds herself in a position where her choices will make or destroy both her own future and that of her nation.
A standalone, action-packed historical adventure from the esteemed author of the Shadows of the Apt series.