The Movement of Crowns — Nadine C. Keels

First in the Movement of Crowns series. 

Is it the perfect or the worst time for the kingdom to...change?
The nation of Diachona rejoices as the king's daughter and heir, Constance, turns twenty. Yet, not everyone approves of Constance's desire to sit on the National Council: a Council run solely by men.

What's more, oppressive threats from a neighboring, powerful empire are rousing the people's fears. Amid rumors of war and personal doubts about her future, Constance suspects that her growing love for one Commander Alexander may be ill-timed. Especially if this is the end of the kingdom as they know it...