Japan has long been a closed country with no trading with the outside. However with the Meiji era (1868-1912) and the beginning of Japan’s modernization policy led by the Emperor Meiji, the country opened up to outside influences and experienced rapid modernization. It was at this time that Japan called UK to assist in its industrial development. England then provides technical and industrial expertise to the Japanese, including in the automotive domain. And as everybody knows of English drive on the left! So the Japanese followed.
According to many people in Japan, the origin of driving on the left would simply find its origin and history … with the samurai. The samurai wore their swords on the left side, so they could easily unsheathe it with the right hand. If two warriors crossed on a road or in an alley, and they had walked on the right, their swords would have rattled, which could be seen as an assault or a provocation.
Because the Romans marched on the left side of the road, it being to draw a sword tethered to your left side, safely, to attack an oncoming army.