The gladius, a short sword commonly used by the Roman legions, was gradually replaced by longer swords as the Roman military tactics and warfare evolved. One of the key successors to the gladius was the spatha.

The spatha was a longer sword, typically with a straight double-edged blade, that started to appear during the late Roman Republic and became more prevalent during the Roman Empire. The spatha was longer than the gladius, ranging from 75 centimeters to over 1 meter in length, allowing for greater reach and more effective slashing attacks.

The adoption of the spatha was influenced by changes in Roman military tactics and the incorporation of cavalry units. The longer reach of the spatha made it suitable for mounted soldiers, providing them with better reach and increased striking power from horseback.

Over time, the spatha became the primary sword used by the Roman cavalry, while the gladius continued to be used by the infantry. The use of the spatha continued to evolve, and by the late Roman Empire, it had become the standard weapon for both cavalry and infantry.

While the spatha was the primary successor to the gladius, it's worth noting that other regional variations and weapons also emerged during the decline of the Roman Empire. These included the migration-era longswords and the Byzantine era swords, which reflected changes in military tactics, cultural influences, and technological advancements of the time.