Oldest swords on record in Japan are the two that were sent as a present to queen Himeko from China during Wei-dynasty in 240 a.d. In 280 a.d. many more iron swords were imported from China to Japan.
It is believed that the art of forging a steel sword came soon after from China and Korea, but the details are unknown. We do know that in the 5th century steel swords were already made in Japan. These were of the straight, single-edged type called chokuto. The method of hardening the steel that is so typical of Japanese swords was first used in 6th century.
The era of the straight sword lasted until the 8th century. Then the predominant style of warfare changed from fighting on foot to fighting on horseback. To accommodate horseback fighting the swords became curved. These long, curved single-edged swords were called tachi. There were many intermediate forms between chokuto and tachi. The most common of these were kogarasumaru (a curved, two-edged sword) and kenukigatatachi. The term Nipponto or Nihonto (literally "Japanese sword") is usually reserved to swords with curvature.
Japanese History and Sword Evolution:
Heian Era (794-1184)
As emperor Kammu came to power, the capital was moved from Nara to Kyoto. The whole era was characterized by the prevalent tendency toward accepting what came through the Chinese influences that had came over the sea during the previous centuries and making them Japanese. Many of the cultural idiosyncrasies that we hold typically Japanese were born in Heian period.
In this era was also created the method of forging a sword with hard outer surface and soft core.
This was the era of tachi. It became customary to sign the blades. The oldest signed blade is probably one tachi forged by Sanjo Munechika. The oldest tachi with date as well as the name of the smith engraved on the tang is from 1159 and was made by Namihira Yukimasa.
During the Heian era two clans, the Minamotos and the Tairas, who rose in power and importance. The end of the era is marked by the battle in Dannoura, where these two clans clashed together.
Kamakura Era (1184-1333)
After defeating the Taira clan at Dannoura, Minamoto no Yorimoto - now the de facto ruler - moved his shogunate to Kamakura. Emperor Gotoba, the formal ruler, remained in Kyoto. This also marked the beginning of the rule of the samurai class.
Kamakura became a cultural capitol, and swordsmiths from all over the country gathered there. Perhaps the most famous of them was Masamune, one of the masters of the soshu-style. The overall style of swords became more flamboyant, more in tune with the newly found power of the samurai. Kamakura period is often held to be the golden age of Nihon-to.
A typical Kamakura sword was wider than before, with little difference in width between the top and bottom of the blade. Kissaki (the point) was often of the type ikubi ("bull's neck").
Late in the Kamakura period came the two Mongol invasions (1274 and 1281) by Kublai Khan. The encounter with the new weapons and tactics of the Mongols demonstrated some weaknesses in Tachi. For example the point was easily broken and could not be repaired. These experiences affected the design of later swords.