But the question is, why have they just now come onto the scene? If dragons have always been around and it's just domestication that is new there is the lag time while they adapt the forces and perfect tactics. In which period opposing forces will learn their own techniques to counter the new technology, just as has happened with chariots, cavalry, artillery - just about every advance in military technology since agriculturs rendered warfare practical. Same difference if it's an advance in sorcery that allows dragon summoning in quantity. About the only way this becomes an instant problem is if there is a fixed portal opens, in someone's territory, Roman or barbarian, just gives different problems, and dragon mounted invaders/refugees/mounted postal service flood through. And Europe or Asia Minor - further away there'd be time to learn, to adapt to the new situation. It's not that a dragon can't fly anywhere faster than the news of its arrival - you need to arrange bases, logistics, a breeding program for the future - years, probably.