Japan only fought one foreign power being the mongolian invasion by kublai khan in 1274 & 1281 both failed due to the typhoons that destroyed the mongolian navy,also mongols had a hard time with japanese terrain, after that they remained somewhat isolated from the rest of the world until the portuguese arrived in 1543.

Japanese imperial army used shields and straight swords in early centuries until the heian period when they switched to mounted archers after defeating the emishis (northern rebel tribes from whom they took the technique) and by the time the mongols invaded japanese infantry used mostly bow and spears or naginata and they switched to curved blades to be able to cut the heavy leather armor mongolian soldiers wore, after that they stuck to that design and style of swordfighting so shields were useless.

Internally japan warfare occurred in the late heian period with the genpei war (1180-1885) which resulted in the establishment of the first shogunate (kamakura in 1192 by minamoto yoritomo) and then the sengoku period (parted in muromachi period 1366-1568, azuchi-momoya 1568-1603 and ending in 1603 with the tokugawa shogunate starting the edo period) as to how they managed to hold their own basically the wealthiest clans were the most powerful as they made campaigns against each other to gain more fiefs, before the portuguese arrived with fire arms, armies were mostly composed of spears and bows with both heavy cavalry ( introduced by takeda shingen) and light mounted archers, after the fire arms, armies were pikemen and musketeers (cavalry went in decay after the battle of nagashino a battle combining forces of oda nobunaga and tokugawa ieyasu who defeated takeda katsuyori) so again shields were useless.