The Katana is a curved, single-edged sword traditionally used by the samurai. According to legend, the Japanese sword was invented by a smith named Amakuni (c.700 AD), along with the folded steel process. In reality the folded steel process and single edge swords had been brought over from China through trade in the early 10th century during the Tang Dynasty. Swords forged between 987 and 1597 are called koto or "old swords"; these are considered the pinnacle of Japanese swordcraft. Early models had uneven curves with the deepest part of the curve at the hilt. As eras changed the center of the curve tended to move up the blade. The katana as we know it today with its deep, graceful curve was developed sometime around the middle of the Heian period (9th to 11th centuries) to service the need of the growing military class. Its shape reflects the changing form of warfare in Japan. Cavalry were now the predominant fighting unit and the older straight chokuto were particularly unsuitable for fighting from horseback. The curved sword is a far more efficient weapon when wielded by a warrior on horseback where the curve of the blade adds considerably to the downward force of a cutting action.
The katana was typically paired with the wakizashi or shoto, a similarly made but tiny sword, both worn by the members of the warrior class. It could also be worn with the tano, an even smaller similarly shaped blade. The two weapons together were called the daisho, and represented the social power and personal honor of the samurai. The long blade was used for open combat, while the shorter blade was considered a side arm, more suited for stabbing, close quarters combat, decapitating beaten opponents when taking heads on the battlefield, and seppuku, a form of ritual suicide.
The Mongol invasions of Japan in the thirteenth century spurred further evolution of the Japanese sword. Often forced to abandon traditional mounted archery for hand-to-hand combat, many samurai found that their swords were too delicate and prone to damage when used against the thick leather armor of the invaders. In response, Japanese swordsmiths started to adopt thinner and simpler temper lines. Certain Japanese swordsmiths of this period began to make blades with thicker backs and bigger points as a response to the Mongol threat.
By the fifteenth century civil war erupted, and the vast need for swords together with the ferocity of the fighting caused the highly artistic techniques of the Kamakura period (known as the "Golden Age of Swordmaking") to be abandoned in favor of more utilitarian and disposable weapons. The export of katana reached its height during the Muromachi period when at least 200,000 katana were shipped to Ming Dynasty China in official trade in an attempt to soak up the production of Japanese weapons and make it harder for pirates in the area to arm.
In the 15th and 16th centuries, samurai who increasingly found a need for a sword for use in closer quarters along with increasing use of foot-soldiers armed with spears lead to the creation of the uchigatana, in both one-handed and two-handed forms. As the Sengoku civil wars progressed, the uchigatana evolved into the modern katana, and replaced the tachi as the primary weapon of the samurai, especially when not wearing armor. Many longer tachi were shortened in the 15th-17th centuries to meet the demand for katana.
The craft decayed as time progressed and firearms were introduced as a decisive force on the battlefield. At the end of the Muromachi era, the Tokugawa shoguns issued regulations controlling who could own and carry swords, and effectively standardized the description of a katana.
Japanese swords are fairly common today, antique and even modern forged swords can still be found and purchased. Modern nihonto or Japanese-made swords are only made by a few hundred smiths in Japan today at contests hosted by the All Japan Swordsmiths Association.