The "strongest" sword ever made is difficult to determine definitively as it depends on various factors such as the specific criteria used to define strength, the historical context, and individual perspectives. However, one sword that is often regarded as exceptionally strong and influential in sword-making history is the katana.

The traditional Japanese katana is revered for its strength and cutting ability. Crafted through a meticulous process of folding and differential hardening, the katana exhibits a combination of hardness, flexibility, and sharpness. The quality of the steel, the skill of the swordsmith, and the unique construction techniques employed contribute to its strength and durability.

Katana blades were traditionally made using a combination of high-carbon and low-carbon steel, resulting in a hard, sharp cutting edge and a more flexible spine. This differential hardening technique, known as "yakiba," created a blade that could retain its sharpness while being able to absorb and disperse impact forces.

The katana's design and construction were honed over centuries, incorporating knowledge and expertise from generations of swordsmiths. The sword's curvature, shape, and geometry were carefully crafted to optimize its cutting performance, strength, and balance.

While the katana is often celebrated for its strength, it is worth noting that the strength of a sword depends not only on the materials used but also on the quality of craftsmanship, design, and intended purpose. Furthermore, historical and cultural contexts play a significant role in the perception of strength and the significance of different swords.