Infinity (Chronicles of Nerissette) by Andria Buchanan:

The kingdom of Nerissette is finally at peace. After multiple battles, Queen Allie just might have the time to start building a relationship with the dragon next door, Winston, before her sixteenth birthday. But unrest is forming at the borders and soon she realizes that it's not just the Fate Maker who wants to take her crown, but her aunt, Queen Bavasama.

When Bavasama sends her troops over the border into Nerissette on a campaign of mass destruction, Allie has to decide whether or not she's willing to bring her battle-scarred country back to war. She can't take the chance of letting more people die, or letting those she's lost disappear from memory.

She must survive long enough to save her country and, in the end, decide what she will do when faced with a final, horrible series of choices.