The following is an excerpt from a history textbook used by the Bardic College for its youngest students, ranging in age from 12-14.
The year 323 ER was unusually hot, and in Martel it came at the end of a bitter succession war. The newest king in Marnes, Adelbert the Third, had been raised as a paladin of the order of Palenous, and had been a successful general in the border disputes with Tintagel and the dismantling of the Brittanwood Bandit Kingdom in the 310s (Side note: Imperial control over its provinces' militaries was not fully established until the early 5th century, and during this period there were some conflicts between provinces). He had won the succession conflict largely because the populace believed in his abilities as a military leader, and so upon gaining the throne he began looking for a conflict to engage in.
Due to the hot weather that year, two things happened: for one, the year saw a dismal harvest as fruit died on the vine and plants withered in the hot sun; and for another, the red dragon breeding season was both enthusiastic and destructive.
As you now live in a world without dragons, you would not have witnessed their mating habits. Red dragons in particular, however, tended to be extremely destructive in their courtship, starting fires and causing numerous casualties. Fires in the southern Brittanwood as a result of this destructive behavior angered the green dragon population there, and the two species fought with each other openly over northern Martel. Thousands died, and the grape harvest was all but ruined by fire, poisonous gases, and a fleeing peasantry.
King Adelbert had already dispatched troops to do what they could to protect the populace, but it was one of his advisers that escalated the situation. Magus Criada, Protector of Marnes and one of the most powerful mages in the Empire, hailed from the Monde Hills northeast of Loire. Her family had been killed in a battle between two dragons, and she openly sought revenge on the powerful creatures. She encouraged Adelbert to strike out decisively against the creatures, driving them back to their respective homes in the mountains and the deep forest, never to trouble the people of Martel again.
Adelbert enthusiastically embraced this mission, but soon discovered that after years of civil war, he did not have all the troops necessary to defeat the marauding dragons. After a half a year of fighting that saw far more dead Martelan soldiers than dragons, he convened--at Criada's suggestion--with the Elders of the Temples, who agreed to help him by sending out a call for assistance from soldiers and adventurers across the Empire. The church of Palenous began referring to it as "The Dragon Crusades", and the name has endured.
When the Mage Academy discovered what Criada had suggested, they brought her before a tribunal and stripped her of her rank and her title of Protector. Criada's suggestions were so far outside the bounds of a Protector's duties that she is held up at the Academy as an example of gross negligence.
The Dragon Crusades spread like the wildfire they was meant to contain. At first it was confined to Martel, and the country's economy strained under an influx of adventurers and soldiers seeking glory. Given that the red dragons of Martel had always been open about their existence, they were massacred, and the green dragons of the Brittanwood did not fare much better. It's thought that some survived, but they must have fled to other lands--no dragon has been seen in the woods in over half a millennium.
In the end, the crusade did not end within the borders of Martel. Adventurous groups made their way to Fianna only to find the druids denying them access to the deep woods, but Bayern and Hohenshau, so focused on fighting each other, did not mind the influx of adventurer coin. Though neither nation had many targets to offer, many crusaders stayed behind as mercenaries for one side or the other. Talar--at that time a very new colony of Arad--was home to a few blue dragons, and those foolish enough to trespass into the dwarven mountains in the north found a number of less intelligent white dragons to slay. The bottomlands of Bevin and Emilia brought in the last great harvest, this time of the vicious swamp-dwelling blacks. By 327, it seemed the Dragon Crusades would be coming to a stop. However, hatred of dragons was going nowhere, and dragon scales had proven a very useful component in the forging of armor. The city-states of Duvrain, in particular, were keen to buy dragon scales by the shipload during this time.
Prior to this, the Crusades could have been seen as a foolish but ultimately helpful extermination of an evil, destructive species. However, while they were not nearly as common, there were a number of metallic dragons in the Empire--good, wise creatures whom our predecessors in the Bardic Guilds, as well as the mages of the Academy, considered friends and great living libraries. Had we done more to speak out against the slaughter of the evil chromatic dragons, that would have been the end of it. Instead, using spells of true sight, priests began hunting for dragons lurking among the Empire's populace in polymorphed form. While they turned up a few reds and blacks in this manner, the majority of the dragons disguising themselves as humans and halflings were good dragons--silver, brass, copper, and gold--doing so out of a desire to observe and assist our still new civilization. This did not stop the swords and spells of the Crusaders, and while the Academy and the Bardic Guilds did what they could to give Metallics safe passage out of the Empire, many more were lost than were saved.
We now find ourselves as a society that thinks very little of dragons. On the southern continent they convene with dragons, and sometimes even allow them to rule city-states or tribes. Here, few people even think of dragons, and those that do tell stories of them as destructive monsters. I for one fear that without their wisdom we will fall behind other nations, both in magic and in culture, and the Eracian Empire will wish it had made allies out of some of its dragons. But for now, all we have are their bones.