Prehistory
Recent archaeological research has uncovered traces of wars as far back as the Jōmon period (ca. 10,000–300 BC) between the various tribes existing on the Japanese Archipelago. Some theorists believe that shortly after the Yayoi period (ca. 300 BC – 250 AD) horse riders from the Korean Peninsula invaded southern Kyūshū, then spread to all the way to northern Honshū. This is when horse-riding and iron tools were first introduced to the islands.
Read more about this topic: Military History Of Japan During World War II