There’s a very interesting answer on Quora to the question “How Much Did Ancient Rome and China Know Each Other?”
6,000 Romans went missing in 53BC. 17 years later, Chinese fought an army with soldiers using roman formation.
Here’s more detail:
The history record reminded researchers of the story floated in the 1950s by an Oxford history professor – Homer Dubs – who proposed the following chain of events that could have brought a band of Roman soldiers to Han Dynasty China.
Battle of Carrhae - In 53 BCE a Roman army under Marcus Licinius Crassus fought and lost a war against Parthian Empire near the town of Carrhae. 6,000 Romans were captured by Parthian and sent to their eastern frontier, but was never found agian.
Battle of Zhizhi - In 36BC, Han army under Chen Tang assaulted a border town known today as “Taraz,” located in Kazakhstan near the border of Kyrgyzstan. Chinese historian noted that: 土城外有重木城 palisades of heavy tree trunks，步兵百餘人夾門魚鱗陣 Hundred infantry in “fish scale” formation 漢軍大勝 Han victory，生虜百四十五人Captured 145 降虜千餘人 Surrendered 1,000+.
The “fish scale” formation is almost certainly the well-known Testudo formation.
That would make a cool story for a movie.