The use of the horse as a means of transportation has been around for centuries. The use of the horse as a domesticated animal in farming and other domestic endeavors has been long recognized as a turning point for civilization as the help that the animals provided allowed people to work greater swaths of land which allowed them to grow more crops then they needed which in turn allowed them to sell or trade that surplus with others for things they required.
The horse as a weapon used in war fighting has also enjoyed a long and storied history. While the use of cavalry as a tool of war fighting can be seen in history as far back as the period of 3000 BC in Mesopotamia, it was not really until the Greek era and the Assyrians that the concept was truly mastered and respected as a viable component to the war effort.
During the Middle Ages, the concept of heavy cavalry was put in effect with great success. The horses and men were both swathed in armor and the charge by a brigade of heavy cavalry was seen as a back breaking attack that alone could change the course of a battle.
While many innovations were seen as making the waging of war more productive, it was the invention of the stirrup that truly revolutionized the use of the horse and rider as a vital component to the effort.
The stirrup allowed a rider to be able to employ a variety of weapons without falling out of the saddle. Without the stirrup, a rider would have never been able to successfully swing a sword or in later years, fire a rifle while riding a horse.
Many battles in the Middle Ages were decided by the cavalry and the success or failure of that arm of the military.