(
Topic should be in 'military history' forum - could somebody move it?)
This is one of those huge subjects that comes up here quite regularly! There are a few previous threads that debate it:
The Abandonment of the Gladius for the Spatha
Segmentata
Why Change to the Spatha?
Curved Shields and Spatha
The most convincing theories involve changes in tactics by the Roman army, probably in response to the changing tactics or military cultures of their enemies. Segmentata seems to fall out of use around the beginning of the third century, but evidence of it exists dating to the late third century. The last examples of rectangular curved scuta date from AD260, and appear alongside flat oval shields. Similarly, the longer spatha appears some time around the end of the second century. This all seems to suggest a major, but perhaps gradual, shift in tactics during the period c180-280.
Could it be that the spear and longer sword were adopted to counter a more commonly mounted enemy? Could the oval shield be better adapted to fighting with a spear and a longer blade?
It might be significant that the spear, mail and oval shield, increasingly used by the legions in the third century, were previously considered the preserve of the auxiliaries (although this could be a modern misconception!). Auxiliaries all gained Roman citizenship under Caracalla; perhaps many of them were drafted into depleted legions, and brought their equipment with them?
Alternatively, one of the better suggestions for the decline of segmentata is that it was very difficult to maintain. With increasingly mobile armies sending vexillations far from base, the more durable mail might have just been more practical.
- Nathan