Her father, the king of a small nation, had ruled with honor for much of his life, but as he got on in years, his mind became paranoid and irrational. His distrust eventually led him to order and unprovoked attack on one of their neighbors, which started an intense and bloody war. Towards the end of the war, he lost his battle with illness and left it to his young daughter to rule the war-torn nation.
With the enemy advancing at every turn and her people suffering endlessly, the queen was left with no choice but to surrender. The king of the other nation, a man only a little older than her, arrived at the palace with a list of demands before he would accept their surrender. He wanted gold, land, and a number of treasures that were dear to the queen's people, but most of all, he desired her. She would come back with him and become his wife, though he made it clear she would have no rights as a queen. She would be a trophy to show how far her people had fallen, a slave. The idea of her sitting on a throne beside him, bound and helpless as he made his judgement, was endlessly appealing to the young king.
During the trip back to her new home, she is assigned a personal guard from her husband's forces, though the guard is mostly tasked with making sure she doesn't escape. He sees her bravery and willingness to suffer for her people, and can't help but want to help her. He starts to offer her small mercies when the king isn't looking, and before long, the two have started a loving relationship in the midst of her cruel new life.
(I"m looking to explore two different relationships here. The queen would be expected to submit to her husband, who would take great pleasure in showing her off as a captive and a slave. There would be a lot of bondage and control in those areas. The rest of the time, I would want to explore a more light and caring relationship between the queen and the guard.)