Blood of Troy by Claire M. Andrews

A year after saving the powers of Olympus by defeating Nyx, the Goddess of darkness, Daphne is haunted by still-looming threats, her complicated feelings for the god Apollo, and the promise she made to the Olympian gods that she would help them again when they called upon her. When their command comes, it is deceptively simple: secure herself a spot as one of Queen Helen’s guards.

A war is coming, and all of Sparta must be prepared. 

In the midst of a treaty summit among the monarchs of Greece, Daphne and Helen uncover a plot of betrayal—and soon, a battle begins. As the kingdoms of Greece clash on the shores of Troy and the gods choose sides, Daphne must use her wits, her training, and her precarious relationship with Apollo to find a way to keep her queen safe, stop the war, and uncover the true reason the gods led her to Troy. But the gods are keeping more than one secret, and Daphne will be forced to decide how far she is willing to go to save those she loves—and whose side she's on in a war that is prophesized to be the downfall of her people.