Determining the "strongest" sword in history is subjective and depends on the specific criteria and context being considered. However, when discussing the strength of swords, several factors come into play, such as the quality of materials, design, construction techniques, and intended use. Here are a few swords that are often regarded as exceptionally strong:

1. Damascus Steel Swords: Damascus steel swords, produced in the Middle East from around 300 to 1700 CE, were renowned for their exceptional strength, sharpness, and flexibility. These swords were forged using a technique that involved layering and folding the metal, resulting in a distinctive pattern and superior strength.
2. Japanese Katana: The katana, a traditional Japanese sword, is highly regarded for its strength and durability. Crafted using a combination of hard and soft steel, the katana's curved blade design and differential hardening technique contributed to its strength, resilience, and cutting ability.
3. European Longsword: The European longsword, used during the medieval period, was designed to withstand heavy combat and had a strong, well-balanced construction. These swords were often crafted with high-quality steel and had sturdy crossguards and pommels to enhance their overall strength.
4. Viking Ulfberht Sword: The Ulfberht sword, used by the Vikings during the medieval period, is known for its strength and superior craftsmanship. These swords featured high carbon steel blades that provided exceptional cutting power and durability for their time.
5. Chinese Dao: The Chinese dao, a single-edged saber, was designed to be a robust and sturdy weapon. Dao blades were often crafted with high-quality steel and had a curved shape that allowed for powerful cuts and strikes.

It's important to note that the strength of a sword is influenced by various factors, including the quality of craftsmanship, the specific design and construction techniques employed, and the materials used. Different swords excel in different aspects of strength, such as tensile strength, resilience, or cutting ability. The perception of the "strongest" sword can also vary depending on cultural and historical contexts, as well as individual preferences and criteria.