I believe this is <PERSON> Fall of a Kingdom, originally published as Flame (2003).
If I remember right all nobles have black hair, and a daughter to a noble or a king was to be sacrificed. The father saved his daughter by hiding her away with a farm couple. She ends up in a desert with short magical people.
“Her loose hair—the straight, black hair of the noblest of noble lines—was disheveled.”
“The commander of the army must sacrifice the being he holds most precious in all the world,” the priests had said. “Or the djinn of war will give their favor to the armies of the Hrum, who will roll over Farsala like the darkness of the pit itself.”
There is also an invading army conquers all lands. All soldiers can speak the tongue of the native people in land they are currently conquering. If I remember right if a country can hold out for up for 10 years they'd be integrated into the Conquering Empire rather than conquered.
“The Hrum try not to let it be known among the lands in their path, but their commanders are given a time limit—one year from the day of the first battle. 164
If at the end of that year they do not control all cities, large towns, and most of the countryside, their army is withdrawn, and negotiations begin for the country they attacked to become an independent, allied state.”
I remember one of the first battles all the black haired nobles were on there Their best horses (even if the horse had white socks, that was bad luck in their culture)
“And though one white sock was thought to unbalance a horse’s beauty, Rakesh wasn’t the only horse who had one. Deghans might be fools about a great many things, but horses weren’t one of them. In battle they rode the best horse, not the most beautiful.”
if I remember right the nobles were so confident in their Cavalry. That all the <PERSON> families watch the nearby hilltops with all their Treasures. They lost the battle and Nobles were captured.
“Knowing the decisive battle was imminent, the deghans had sent for their families, “to witness and share in the glory, according to our ancient tradition.” It was traditional for a deghan’s family to accompany him to the field, bringing with them as much of the household’s treasure as they could reasonably—or sometimes unreasonably—carry. This was supposed to show confidence in the army’s invincibility, <PERSON> supposed.”
Fall of a Kingdom is the first of the three volumes of The Farsala Trilogy. 351