Inspired by this question, I am wondering something similar. In a modern war a dragon that simply flies over enemy forces and torches them with its fire breath there are several ways of taking it down with aircraft and anti-air weapons.
But in earlier history I know this would not be the case. A big flying thing that breathes fire and just shrugs off arrows and musket shots because of its thick hide would be a game changer for nearly every battle I can think of. Rome would rise or fall in a matter of years, the Golden Horde would be even deadlier, the Alamo would be remembered because a dragon was there, it could melt stone walls and Stonewalls to shorten the Trojan and Civil Wars, Waterloo would be Fireloo, the Kettle War would have to be renamed because a lot more than just a kettle of soup was hit and so on.
But my question is: when is the turning point where the inclusion of a dragon does not mean an automatic victory for the side it is on? Would this be around World War I, with the Red Baron taking it down? Or perhaps World War II with its many and more powerful aircraft? Or maybe later still?
Note, whatever the situation it is the first time a dragon is deployed in war, so combatants have to figure out on their own how to kill it.
Notes on the dragon:
- The dragon is the same size as Smaug as he appeared in the movie adaptation of The Hobbit.
- The dragon can breathe fire, and it uses this as its primary form of attack when claws are not a better idea for whatever reason.
- The dragon is willingly aiding one of the sides and acts to the best of its abilities.
- The dragon is capable of discerning friend from foe and will not intentionally cause friendly fire.
- The dragon is clever enough to think of a way to counteract any attempts to shoot it down that are in progress.
- Small arms fire cannot penetrate its scales, which cover all of its body.
- When met with a way that could reasonbly kill it before the dragon can destroy the attacker it will attempt to retreat to formulate a plan or call for backup.
- The dragon is willing to fight and risk possible injury, but it will not risk death.
- If the time period allows it, the dragon is outfitted with communication devices that allow HQ to feed it intelligence.
- The dragon has received training in non-electronic communication devices and is aware of locations HQ can send it messages from (signal flags, sound-based communications et al).
- The dragon has senses on par with the most potent senses in the animal kingdom. It can opt to lower their sensitivity to be on par with that of a human to prevent it from being blinded, deafened or otherwise disabled by impulses that would cripple delicate senses, but not that of a human.
- The dragon has no hoard or family members with which it could be blackmailed into desertion/treason/whatever.