Ennnhhh… I’m not sure about this, Steven. In fact I’m willing to say its actually wrong.
The Dothraki I don’t think ever had direct contact with the Freehold, and were certainly never engaged in direct military conflict with it. They were kept penned into the far eastern grasslands by three world powers, none of them Valyrian; the Ghiscari city-states, the Qaathi city-states, and the Kingdoms of the Sarnor. The Dothraki existed well outside the Freeholds direct sphere of influence, not even bordering any of their directly conquered lands, and if the Valyrians ever flew dragons against them it isn’t recorded in any source I know about. (Willing to be wrong here!)
Indeed, the Dothraki were such a nonentity that the Sarnori explicitly regarded them, according to WOIAF, as “little more than a nuisance.” They did not rise to prominence until well after all the Valyrian dragons were dead, throwing the Sarnori down when they unified under the first khal of khals.
I don’t think one can reasonably make the case that it was the Valyrians holding the Dothraki back. Other world powers, sure, but not the Valyrians.
Point of order, the Sarnori did not entirely separate the Dothraki from the Valyrians:
“In short, the names and numbers of the peoples who fell to Valyria are unknown to us today…A few, such as the Rhoynar, lasted against the tide for centuries, or even millennia. The Rhoynar, who founded great cities along the Rhoyne, were said to be the first to learn the art of iron-making.
Also, the confederation of cities later called the Kingdom of Sarnor survived the Valyrian expansion thanks to the great plain that separated one from the other … only for that plain and the people who occupied it—the Dothraki horselords—to be the source of Sarnor’s downfall after the Doom.”
You can see this on the map:
While much of the Dothraki Sea is to the east of the Sarnori cities, it does stretch south and west to the Painted Mountains of northern Valyria. And of course, when the Valyrians had conquered the Ghiscari Empire to their east, they would have had the Dothraki to their nrothern border there too.
As for the Dothraki and dragons, it is noticeable that they never attacked either the Ghiscari, the Qatheen, or the Free Cities until after the dragons were gone. And then there’s this:
The one thing the Dothraki fear are dragons, the one thing that held them back from their manifest destiny for thousands of years was the white-haired, purple-eyed dragonriders.
It’s not about power politics for Drogo, it’s about symbolism.
“And now the stone was gone and she flew across the Dothraki sea, high and higher, the green rippling beneath, and all that lived and breathed fled in terror from the shadow of her wings.“ (Dany IX, AGOT)