“It’s amazing how far the reach of the Roman Empire extended. There are stories of Chinese armies using Roman formations (having been trained by captured Roman soldiers).”
It has been postulated that Romans captured by the Persians were used to garrison/patrol the Persian eastern frontier.
It is a possibility. It’s also not unlikely that the Persians/Parthians used the phalanx as a consequence of having been influenced by (and for a long while ruled by) Macedonians and Greeks — in which case they could have been survivors of or deserters from a Parthian (or Seleucid et al) defeat vs the Chinese.
http://www.freerepublic.com/focus/chat/3658793/posts?page=16#16