Princess Maritza

Desmond Ellery is a disgraced English soldier who on a walk one day meets the young Princess Maritza, playing truant from the finishing school she attends. She inspires him by showing her sass and verve, and he decides to go to her kingdom Wallaria and seek fame and fortune there. He gets wrapped up in that countries tumultuous politics, and finds himself in the centre of everyone's plots. (Goodreads)