Carcosa is the name of the 'homeland' of the dragons, but you would be hard-pressed to find the bulk of the world's dragons in it. Massive, ill-tempered, destructive, greedy, and prone to vanity, dragons could never all settle in a single nation, no matter how large- and Carcosa is not particularly big, area-wise. Instead, dragons are spread out across the world, living in every land, on every continent, in every country, among every other race.
Carcosa functions less as a full-blown homeland and more as a jumbo-sized capital city for the vast dragon nation, whose 'claimed territory' is anywhere a dragon is making his or her home. It is the seat of the Dragon Emperor, where he looms in hot darkness and communicates to his empire in ways no one fully understands. It is also the primary breeding ground of all dragons. While there are other 'migrations' all around the world, it is to Carcosa that nearly every dragon treks to truly mate and foster hatchlings. At all other times, perhaps only ten percent of the world's total dragon population resides in Carcosa. In fact, there are more dragons in Equestria than in Carcosa on any given day.
The dragons have twice attempted to take advantage of these numbers and conquer Equestria for their own. Both times they have been defeated. The first time, some 1400 years before the events of Friendship is Magic, actually served to bring Equestria closer together, jarring loose some of the barriers that still separated the three pony types as they were led by Celestia and Luna. It was a loose confederation of dragons that they defeated, with Luna serving as primary general and commander of the Equestrian forces. Though he had not officially sanctioned the campaign, the emperor at the time, Scatha the Frigid, nevertheless agreed to a ceasefire. Its terms were loose, its language tentative, but it served to keep the dragons and ponies at peace for 800 years.
This peace was thrown out 600 years before Friendship was Magic. This time the emperor, Glaurâg the Wingless, personally launched the attack, using the dragons already in Equestria to wage a rebellion that burned countless towns, cost countless pony lives, and even threatened Canterlot itself. However, this conquest attempt also met with failure, and this time all the more spectacularly. Led by a newly self-confident Princess Celestia, the combined forces of Equestria, Gildedale, the Buffalo Tribes, the Shimmerwood, and the Deer Elders crushed the uprising and drove the dragons back across the Simmering Sea. The final battle of the war took place on the volcanic soil of Carcosa itself, where Celestia forced a peace treaty after personally slaying Glaurâg in open combat.
But that's a story for another day.