Origins of the spatha date to the late Celtic Bronze Age in Europe and the use of full-length bronze cavalry & charioteer swords by Celtic warriors during the Hallstatt era. These gave way to first iron, then the piled-rod composite iron/steel long swords of the La Tene era circa 480 BC. By the 3rd century BC, the Celts were using pattern-welded long swords in Europe that would continue in use through the Celto/Roman era and eventually into the later Migration era [Pleiner (1993) The Celtic Sword]. Long-established sword making centers typically produced spatha sword blades in lots that would then be sold to the various recipients for final hilting. For this reason, it is often difficult to distinguish the weapons used by the Romans from those of their adversaries without some other contextual basis to go by, such as other artifacts in a tomb or burial, etc. Also, the Romans made extensive use of Celtic and later Germanic cavalry conscripts and mercenaries who often retained use of their traditional weapons while on campaign - even though they were serving the Roman army.
Of completely unknown origin, the spatha of literature appears in the Roman Empire in the 1st century as a weapon of presumed Germanic auxiliaries (whether infantry or cavalry is not known) and went on from there to become a standard heavy infantry weapon, relegating the gladius to use as a light infantry weapon. There is no evidence that the spatha was used exclusively for slashing. It apparently simply replaced the gladius in the front ranks, giving the infantry more reach in thrusting.
Archaeologically many instances of the spatha have been found in Britain and Germany. It was used extensively by Germanic warriors but whether it came from the Pompeii gladius or the longer Celtic swords or served as a model for the various broadswords and Viking swords of Europe is a highly speculative topic. The spatha remained popular throughout the Migration period. It could have evolved into the knightly sword of the High Middle Ages from about 1100, but the large number of sword types that appeared during the period are difficult to connect for certain. Specific details of their manufacture and the models used by their manufacturers remain chiefly unknown.
Employed by both Roman cavalrymen and their German enemies, later Lombard spathae were actually more advanced than the wrought iron gladii, being constructed using a form of pattern welding employing layers of iron and steel: in effect, a composite material. Eventually under the later Roman Empire the spatha was adopted by many if not most legionaries. The Latin word, spatha, and all its derivatives, are a loan from ancient Greek spathe (σπάθη), any object considered long and flat, such as the blade of an oar, a rib, the shuttle of a loom, a spatula, and so on. Also in ancient Greek culture spathe was used in the middle Archaic period (no earlier than the 6th century BC) for various types of Iron-age sword, appearing as early as the works of Alcaeus of Mytilene (Chalkidikai spathai, Alc.15.6). As far as can be known now, it does not reach back into the Bronze Age and does not appear in the works of Homer.
Eventually spathe was adopted by the Romans under the Roman Empire in the general sense as spatha. It appears first in Pliny and then Seneca with different meanings: a spatula, a metal-working implement, a palm-leaf and so on. There is no hint of any native Roman sword, spatha.
As a sword it first appears in the pages of Tacitus with reference to an incident of the early empire. The British king, Caractacus, having rebelled found himself at last trapped on a rocky hill, so that if he turned one way he encountered the gladii of the legionaries, and if the other, the spathae of the auxiliaries. Left with no successful way to turn, he escaped to the Brigantes, leaving his brothers to surrender the men, was turned over to the Romans by the queen of the Brigantes, was pardoned by the Senate after a moving plea for mercy, and reigned successfully once more as a Roman client king.
Tacitus does not relate who the auxiliaries were. The Romans moved auxiliaries around the frontiers and also relied on local levies. Most examples of spathae come from Germany and east Europe, however. There is an excellent chance that the owners of the spathae were Germanic. There is no indication in Tacitus either that they were cavalry; overall, the Romans used both cavalry and infantry.
When next the spathae appear, after a mysterious lacuna of about two centuries, they are the standard weapon of heavy infantry. The Romans evidently borrowed this weapon from the auxiliaries, probably Germanic mercenaries, but the name gives no indication of that origin. The etymological dictionaries tie it English spade, spoon, spatula, and so on, while Julius Pokorny and others based on Pokorny give the root as *spe- or *sp(h)ə-(dh)-, meaning a physical implement, which the etymologists conjecture was flat and wide.
Spatha was certainly not a Germanic name, nor is there any indication anywhere what its Germanic name was. There are a plenitude of Germanic names, such as Old English sweord, bill, and so on, but no evidence to tie any name to the spatha, which was never used in Germanic as the name of a sword. English adopts the other uses: spade, spatula, and so on, but nothing like the Italian, French or Spanish words.
Perhaps the most distant recognizable cousin to the spatha were the Viking age blades. These swords took on a much more acute distal taper and point. These blades had deep fullers running their length, yet still had single-handed hilts which sported a unique shaped pommel, flat at the grip side and roughly triangular early on, with the flat curving to fit the hand better later. While the pattern of hilt and blade design of this time might readily be called 'The Viking sword' to do so would be to neglect the wide spread popularity it enjoyed. All over continental Europe between AD 700 – 1000 this design and its small variations could be found. Many of the best blades were of Frankish origin, hilted in local centers. The balance is significantly better. Many Saxon era blades were largely ceremonial, due to the low grade of iron and the tip-heavy balance. Viking era blades were refined weapons.
During "Norman" times the blades increased some 100 mm (4") in overall length, and the hilt changed significantly. Instead of the Brazil-nut pommel, a thick disc-shaped pommel was attached 'on-edge' to the bottom of the iron hilt. In addition the upper guard grew substantially from the near-absent design predating it. Also the blades tended to taper slightly less than those found during the times of the Vikings.
Jan Petersen in De Norske Vikingsverd ("The Norwegian Viking Swords", 1919) introduced the most widely-used classification of swords of the Viking Age, discriminating 26 types labelled A – Z. In 1927 R. E. M. Wheeler condensed Petersen's typology into a simplified typology of nine groups, numbered I – IX.
The transition from the Viking age spatha to the High Medieval arming sword takes place between the 10th and 11th centuries. The main development is the growth of the front handguard into a full cross-guard, and the reduction of the typical Viking Age lobated pommel into simpler hazelnut or disc shapes. The sword of Otto I preserved in Essen is such an example of the emerging arming sword, although it has been encrusted with decorations during the centuries it was conserved as a relic (total length 95.5 cm)