This is one of the lesser-known novels for teenagers by Frances Hodgson Burnett (best known for 'The Secret Garden'). It was written about a hundred years ago, so is out of copyright and was free for my Kindle.

The story is loosely based on fact, but features an imaginary Eastern European country called Samavia. Marco, the young protagonist of the book, has been brought up as a patriot by his father, despite never having been there. He is observant and intelligent, and knows when to be quiet. He becomes friendly with a disabled street urchin known as 'the Rat', who eventually becomes his aide-de-camp when the two boys, in a somewhat unlikely scenario, travel across Europe to give a 'sign' that will start a revolution in Samavia.

The eventual ending was obvious from fairly early in the book - I assume the reader is meant to know, as there are so many clues - and there's rather over-much description and philosophising for modern tastes. Still, it's an exciting adventure, if one doesn't mind suspending reality slightly more than usual for a non-fantasy book, and surprisingly modern in much of its outlook.