I don't really have orders, since countries take a long time to get anything done, far more than a day, at least to decide and the like. Though once they set their mind on something...shit gets done!
I'm going to propose ideas and questions for how the world might work, if dragons have always lived with us.
First, why has civilization only been around for 4000 years? I would think, with the addition of dragons, you might want it to take LONGER, rather than shorter, due to how long they live. Especially considering they would have 'discovered' fire a lot sooner...would have had less difficulty getting around...in all honesty, civilization probably started with dragons, and some weird hairless apes did the 'monkey see, monkey do'.
Now, dragons don't have as much of a need for clothes (though I suspect their wings would be relatively sensitive. Dragon sized cloaks.), and it's difficult to make shoes for one(though they probably wear them indoors in the modern times, so as not to let their claws break things. Unless they're retractable.), but cooked food is always a plus(they are NOT immune to disease!), and housing yourself from sandstorms and/or blizzards is pretty much a good idea regardless of species. I am going by a scientific approach to dragons, so fire-breathers have heat tolerance and fire retardant saliva, but they aren't tiny volcanoes, and cold dragons probably breath some form of liquid nitrogen and have incredibly well-insulated scales. Doesn't mean a blizzard isn't dangerous.
So consider when humans and dragons would first meet. Probably as soon as they saw us, and thought: "Hmmm, food." When we first stopped killing each other? Probably when we met a dragon, didn't try to fuck with it, and it wasn't hungry or cruel. Civilization would have dragons involved, partially because humans wouldn't all hate them unitedly. There would be some humans trying to negotiate, some wanting to run, some wanting to fight. Same thing with dragons, except instead of run, it's more 'don't directly mess with the intelligent apes that know how to dig and have pointy sticks'. The humans and dragons who got along would have done far better than those who don't. Humans can't fly, or breath fire. Dragons don't have very good manual dexterity, or the ability to do things that require exertion for long periods of time(they're like cats. Lots of energy in a short time; not much endurance, proportionally).
Dragons, being sentient, would probably hate to be ridden, and to be honest, they wouldn't really need to be. Having a dude on top doesn't help in combat. Especially not when you could hold the goddamned lance and/or ballista yourself. But there would be something to counter them. Their scales are hard; not impenetrable. Bows souls be meant more for hitting and going through scales, meant to counter dragons first, and people second. You can't really beat a dragon in hand to hand, unless you're another dragon. You can make it a pincushion. And so tactics wouldn't create the lovely tit formations and close combat, that breath weapons just love. It would be far more about sieges, skirmishes, and each side trying to keep their side occupied and hopefully win. In the air, if the dragons win, suddenly you have five ton scaled flying beasts trying to kill you. On the ground, if the humans win, suddenly you have four hundred people with rather powerful bows launching sheets of metal-tipped pointed sticks at you. Siege weapons would be a laugh. Needed yes, but they'd be meant to be heavy, mobility wouldn't be an issue(you can't outrun a dragon), but they'd likely be as well armored and fire resistant as possible(assuming the ones we met breathed fire as a majority), and heavy to prevent picking them up and dropping. Guarded too, and preferably hidden as much as possible. This all translates to what happens with modern times. Tanks are a natural step from this, meant to fight dragons. Fast enough to keep up, somewhat at least, armored enough that claws can't get through, heavy enough that they can't be thrown or dropped. Combine this with rapid firing large caliber(by small arms standards; think 25 millimeter) guns, and you have AD. Humans like flying too, and everyone likes getting better, so flying harnesses, and eventually, propellers for dragons, would develop. Planes would exist, mostly as bombers and transports. A fighter plane is a lot faster than a dragon. A dragon is also infinitely more maneuverable. The dragon will win. There'd be a brief experiment, which would fail, before it goes to just dragons in special harnesses armed with machine guns and missiles. Probably wearing special armor. Maybe metal claw things. Bombers would exist though, and the machine guns would be meant to kill interceptors(which would be the only fighter-type of plane really in existence, just to counter bombers) and dragons. To counter tanks, some tanks might carry another gun with true large caliber shells, there might be tank destroyers, etc. Artillery would go nowhere without accompanying AD weaponry. Infantry would still carry battles through though, as a heat seeking missile works a lot better when you breath fire. Guns would simply be less likely to carry hollow-point bullets, and far more likely to carry SP(scale piercing). Just in case.
This all means nothing without mechanics to back it up, so, in general...
Tank --> Dragon --> Light Armor --> Infantry --> Tanks.
Dragons ~ Infantry, Light Armor ~ Tanks.
Dragon --> Fighter --> Bomber --> Dragon(they aren't fast enough to close before the MG tears them apart)
Definitely less sure how sea battles would work. Aircraft carriers would carry bombers, and they'd also be known as Dragon Transports. You can guess why.
Though, Dragons make excellent dive bombers. They can just throw the bomb as they dive. Probably multiple, if they have a satchel.