Inevitably, at some point, someone decided that horses would be useful in warfare and battle and historical etchings in Sumeria dating to 2,500 Before Christ show horses being used to pull loads into battle.
But while the first use was simply in terms of load bearing labor, they were quickly trained to be used to pull war chariots which normally housed a driver and an archer. Improvements in harnessing and tethering led the Egyptians to raise an army that was almost invincible due to their horsemanship and driving tactics.
In 360 b.C., an ancient Greek army officer wrote the first formal training manual relating to chariot warfare and although this phased out with the rise of the Roman Empire, emperors and generals still rode in chariots which were regarded as a status symbol. While the people of the Far East and Eurasia quickly developed the bit and reins, Europe and the Roman Empire lagged behind.
This is evidenced in the way Asian countries organised races to see who was the best horseman by riding bareback, or playing a very early version of polo, while the Romans stuck to chariot racing. In Rome, although the stones are long gone, you can still see the outline of the Circus Maximus, where races were held.
In the Medieval years, horses were used by the Knights and were also covered by armor to counter arrows fired by bowmen. To this end, they were bred to be huge and also very placid to obey commands in the face of danger.
The warhorse continued to develop over the centuries into a faster but lighter animal that could carry cavalrymen or lancers into battle, slashing and impaling foes and also trampling defenders under foot. Horses were also used in the World War I and II to pull guns and artillery pieces to the field of battle, as well as to transport troops.