This is the first book of Rawn's "Dragon Star" trilogy, the follow-up to her "Dragon Prince" trilogy.
In the first trilogy, High Prince Rohan established and then maintained his rule over the lands. His systems of laws and trade and treaties and marriages have made for a peaceful world. With the exception of Prince Andry and his school of Sunrunners, Rohan has no opponents. And even Andry's rebellion is not one of arms, it is simply one of rival power bases.
All of this changes when a mysterious and seemingly unstoppable invading horde swarms across the lands. Rohan and his son Pol lead the lands in their fight against the enemy, and even have to fight a few princedoms who throw in with the enemy. But their plans, horses, and men, even with the aid of magic and dragons, have trouble rolling back the incursion.
Rawn writes about military strategy in a way that is easy to follow, yet complex and character-based. Her world is so grounded in a sense of place that it it logical for the different princes to have different priorities in the battle, and even different preferred methods for fighting the war. The huge cast of characters feels natural, and the way Rawn handles them is very skillful.
The last two pages of the novel are a terrific (pardon the cliché) game-changer.I have the next book in the trilogy, and will start reading it not too far in the future.