馬が戦争に用いられ始めたのはいつ？
The first use of horses in warfare occurred over 5,000 years ago. The earliest evidence of horses ridden in warfare dates from Eurasia between 4000 and 3000 BC. A Sumerian illustration of warfare from 2500 BC depicts some type of equine pulling wagons. By 1600 BC, improved harness and chariot designs made chariot warfare common throughout the Ancient Near East, and the earliest written training manual for war horses was a guide for training chariot horses written about 1350 BC. As formal cavalry tactics replaced the chariot, so did new training methods, and by 360 BC, the Greek cavalry officer Xenophon had written an extensive treatise on horsemanship. The effectiveness of horses in battle was also revolutionized by improvements in technology, including the invention of the saddle, the stirrup, and later, the horse collar.
between 4000 and 3000 BC