Horses have been used as mounts for the military since early history. The horses had to be obedient and maneuverable, therefore a system of training was developed, first documented in the writing of the Greek master, Xenophon. The system of training was developed throughout the ages, with many well-known riding masters, writing books further clarifying their methods. 

Heavy horses carried the knights of the middle ages in full armor. As modes of warfare changed, the type of horse changed with it, giving way to the lighter horse used for the cavalry. The hot blooded breeds, such as the Arabian and Thoroughbred, were introduced to add swiftness and greater maneuverability to the cold blooded, heavy horses of the armored knights. The resulting “warmbloods” formed the basis for most of the breeds most often successful in dressage today. 

The horses of centuries past were used primarily by the military. The military horse became the standard during the inception of the modern Olympics. The military test included obedience and maneuverability, what would become the dressage test, as well as the ability to jump obstacles. 

Separate studbooks in countries throughout Europe were maintained by the local lord or prince, with the result that many of these warmblood bloodlines can be traced back through a surprising number of generations. Arabian and Thoroughbred lines have continued to be used to further refine the warmblood that we know today which is a leggier, more elegant horse, and with increasingly extravagant movement. These modern-day warmbloods predominate in international dressage competition.

By 1912, the equestrian disciplines as we know them (dressage, jumping, and eventing) were included. However, the riders continued to be all male and predominantly military for a several decades. The United States Cavalry at Ft. Riley exchanged ideas and instructors with the schools in Europe and started the trend that brought dressage training not only to the military but to civilians in the United States. 

Once the US Cavalry was disbanded in 1948, the focus for dressage shifted from military to civilian competition and the sport began to gain momentum. Women as well as men became passionate about dressage. In 1952 the first women were allowed to compete in the Olympics.  

More about dressage in the USA on the United States Dressage Federation website.