The Roman Spatha was a type of weapon used primarily by Roman cavalrymen, hired mercenaries, and later, the Roman infantry. It is longer than a Gladius and about the size of a medieval sword. It allows the soldier to hit both ground fighters and other calvarymen.
Use in Classical Antiquity
The Spatha, as with the Gladius, was adopted from the Celtic and Germanic people of Europe and eventually adapted to suit their purposes. While seeing relatively little service during the Kingdom and Republic of Rome, the Spatha became more commonplace among the heavy infantry of the legions and auxiliary Roman cavalry as (from the mid to late Empire) more foreign mercenaries and soldiers (mostly Germanic of origin) were bought into service for Rome. During the Empire's late years, the Spatha became the main armament of the Roman legionaries, replacing the shorter gladius that had been used for countless centuries before.
The Spatha was offered a combination of both good reach and the ability to be used in a relatively compact area of combat. When used by the infantry, the Spatha provided a greater reach than the Gladius, thus not requiring the infantry to have to come too close to the enemy in order to attack them in melee (enemies could be kept at a distance). While heavier, the Spatha provided the same functions as the Gladius, able to be used for thrusting and slashing alike.
Previously, the main armament of the Roman cavalry had been a medium-sized spear called the Hasta. However, the spears and lances of the Alexandrian age were more suited for use in cavalry shock tactics, where a charge from these lancers/spear-armed cavalrymen from the front would disrupt the enemy formation and cause them to rout with one charge or repeated charges. However, as the enemies of Rome (especially Germanic ones) adopted similar equipment and armour to those of the Romans', enemy units began to hold for longer periods of time, rendering these shock tactics relatively useless as the lances were very cumbersome in extended melee combat. As a result, Roman cavalrymen were armed with the Spatha in preparation for extended periods of combat with infantrymen on the ground or enemy cavalrymen.
Continued Use after the fall of the Western Roman Empire
The Spatha continued to see service in the Eastern Roman Empire, now called the Byzantine Empire. Variants would also be adopted by the various Celtic and Germanic people of Europe (e.g. the Vikings) and it is quite plausible that the Spatha was the predecessor of the Medieval knight's sword.