Sword moving from right to left

[Nathan Ross]

I doubt anything like a '167 AD' directive went out to all legions to begin oval shield and spatha operations.

All these changes (sword right-left and short-long, shield rectangular-oval, pilum to spear) relate to what Simon James (in Rome and the Sword) calls the 'Antonine Revolution' in Roman arms. Whether it was a revolution, or more of a slow evolution over the last quarter or so of the second century and into the third, is unclear. The old curved rectangular scutum still turns up at Dura Europos, mid-3rd century, after all (and on the Arch of Galerius c.300, in the hands of the enemy!).

Either way, I very much doubt that it was the work of a single emperor, or a single 'directive' - more a combination of various ad hoc tactical alterations finding widespread adoption.