The notion of a "strongest" sword is subjective and can depend on various factors. However, it is important to note that the katana is renowned for its exceptional craftsmanship, cutting ability, and durability. Here are some reasons why the katana is often regarded as a strong sword:

1. Blade Construction: Katanas are traditionally forged using a combination of high-quality steel and specialized forging techniques. The blade undergoes differential hardening, creating a hard, sharp cutting edge while maintaining a more flexible and durable spine. This construction method enhances the strength and resilience of the blade.
2. Cutting Ability: Katanas are designed for effective cutting and slicing. The combination of a sharp edge, curved blade, and optimized balance allows for precise and efficient cuts. The shape and profile of the blade contribute to its cutting prowess.
3. Durability: The craftsmanship and materials used in katana construction contribute to its durability. The careful selection of high-quality steel, the differential hardening process, and the overall design make katanas capable of withstanding significant stresses during combat or practice.
4. Historical Use and Testing: Katanas have a long history of battlefield use and underwent extensive testing in combat situations. The design and materials used were refined over centuries to meet the demands of samurai warfare. The katana's strength and effectiveness were proven through real-world combat scenarios.
5. Symbolism and Cultural Significance: The katana holds deep cultural and historical significance in Japan. It represents the samurai's honor, skill, and adherence to the code of bushido. As a symbol of the samurai class, the katana embodies qualities such as strength, discipline, and martial prowess.

While the katana is revered for its strength, it's worth noting that the performance and strength of any sword can be influenced by factors such as the skill of the wielder, maintenance and care, and the specific construction and materials used. Additionally, different types of swords may excel in different aspects, such as thrusting or defensive capabilities.