First, dressage came from the parade ground, not the battle ground. A rider using two hands on the battleground is darn near worthless to the army, and a horse moving with a high degree of collection moves too slow and is too exposed to be useful in battle. But on the parade ground, it looks great.
Second, dressage is not defined as "any riding where a horse uses its rear end". All riding needs the horse to use its rear. That is why horses have big butts. But not all riding is dressage riding. "People need to stop thinking of dressage as some snooty sport that only belongs to one elite group of people and remember that its the oldest form of modern day riding that we have and pretty much everything else has branched off and evolved from it..."
Historically wrong. The Mongols didn't ride or train in dressage, and neither did the American Indians. Neither did the cowboys or cavalry. Neither did the ancient Greeks.
Dressage is a variation that evolved at the same time other variations evolved. It isn't the way the Chinese rode, or the Romans, etc. Dressage is not the Mother of Horsemanship. Other styles didn't branch off of dressage.
If you actually look thru history, and look at saddles and equipment and riding manuals of armies, you don't find dressage. You find something much closer to this:
Look at this image from the 1300s:
That isn't dressage.
It has more in common with the cowboy than with Anky.
Athens, 440 BC:
I believe this postage stamp was derived from a painting done around 600 AD:
This picture is from a tomb carved in the 600s - notice the stirrup placement:
Roughly 900 AD:
From École de Cavalerie in 1729:
Dressage evolved in Europe around 1600-1900, but the main change is a move away from what cowboys used was in the mid-1800s. While both dressage and western riding use a traditional seat, what we see in dressage now reflects an evolution made to adapt to the use of collected gaits in the 1800s. Since cowboys didn't use the collected gaits developed in dressage, they retained the older, traditional seat and leg position.