The Third Prince and the Enemy's Daughter - Calliope, #2.5

Saving your people shouldn't mean them losing themselves. Roger, Third Prince of Congu, has spent more than a year trying to bring what he learned in Anglia to his people. When he considers returning to Anglia, an old conversation inspires him to try one more time. He sets off a chain of events which results in a young woman building a steam engine like her father's, and Roger being sent as a herald to the Congu's traditional enemies.
Both the Sombi and the Congu are at risk of losing their countries to the Kershian Empire. When new enemies arrive, old enemies may be just what is needed.