Japanese swords, specifically the traditional katana, typically require the use of two hands due to several factors related to their design, purpose, and combat style. Here are some reasons why a Japanese sword is wielded with two hands:

1. Blade Length and Balance: The katana typically has a long, curved blade that measures around 60 to 75 centimeters (approximately 23 to 29 inches). The length of the blade, combined with its curvature and weight distribution, provides a proper balance and center of gravity that allows for effective control and maneuverability. Using two hands on the hilt helps the wielder maintain balance and control the sword's movements.
2. Two-Handed Grip: The two-handed grip on the hilt provides stability and power during strikes. By gripping the hilt with both hands, the wielder can generate greater force and leverage for cutting, slashing, and thrusting techniques. This grip also offers enhanced control over the direction and angle of the blade, allowing for precise and fluid movements.
3. Martial Arts Techniques: The use of two hands on the sword is integral to the specific martial arts techniques associated with Japanese swordsmanship, such as kenjutsu and iaido. These martial arts emphasize fluid movements, precision, and a strong connection between mind and body. The two-handed grip facilitates the execution of various techniques, including drawing the sword from the scabbard (saya) and performing intricate cutting sequences.
4. Defensive Maneuvers: In addition to offensive techniques, the two-handed grip on a Japanese sword enables effective defensive maneuvers. By using both hands to control the sword, the wielder can block or parry incoming attacks more efficiently and maintain a balanced defensive posture.
5. Symbolism and Tradition: The two-handed grip on a katana has cultural and traditional significance in Japanese swordsmanship. It reflects the disciplined and structured approach to combat and the reverence for the sword as a symbol of samurai culture and honor.

It's important to note that not all Japanese swords require two-handed grips. Some shorter swords, such as the wakizashi or tanto, are designed to be wielded with one hand or a combination of one and two hands. The choice of grip depends on the specific type of sword, combat technique, and personal preference of the practitioner.