The basic setup will be quite simple: after ruining the country's plans to set up proper anti-dragon defenses in the capital, he will land on top of the city hall and tell them that he is bored of random raids and now demands the respect and tribute that a dragon deserves. Namely, one million gold pieces and 20 of "their young" per month. First delivery expected in 15 days.

Diplomacy would be viable, if the party could somehow convince the country to give up a significant chunk of their GDP and 20 children per month. But I doubt they would do that.

After which, they will have those two weeks to cross the frozen northern wilderness (a hex crawl with 100 traversable tiles) and confront the dragon directly. There are a number of set encounters and locations on those hexes (the most notable is Clawhold, the only permanent settlement of the Tabaxi tribe who populate the north, and the 13 abandoned mountaintop fortresses that they might come back and clear out even after they take care of the dragon). The dragon himself is nesting in the far northeastern corner of this map (the party starts from the southwest, obviously) in a mountain named the Pillar of Lights.