From what I understand, the Odrysian kingdom became a client state of Rome after the Third Macedonian War (171–168 BC).
Did the Thracians (Odrysian or otherwise) ever clash with the Romans in a pitched battle? And if so, did the Thracians use the Rhomphaia weapon against them? Or did they make use of phalanx unit formations?
Plutarch in "Aemilius Paulus" mentions tracians (allied with macedonians) in the Battle of Pydna.
First marched the Thracians, who, he himself tells us, inspired him with most terror; they were of great stature, with bright and glittering shields and black frocks under them, their legs armed with greaves, and they brandished, as they moved, straight and heavily-ironed spears over their right shoulders.
In Russian translation of Plutarch this sentence contains the word "swords" not "spears". And the comments especially insist on rhomphaia being mentioned here.
I can't find anything about the Rhomphaia, but the similar Falx was used in battle against the Roman armies during their subjugation of Dacia:
The Falx forced the only documented change in Roman armour brought about by an encounter with a new weapon. After encountering the Falx in Dacia, the Romans added extra reinforcing bars to their helmets to protect against the powerful blows of this weapon. - Wikipedia
That is very likely I would say almost 100% But unfortunately we have no written sources that can confirm this. Since many Thracians joined the army of Macedon against the Romans that probably happened but since the phalanx were the majority of the troops the authors are not mentioning it. What the person above said about the Dacians is also correct and documented. So the real answer is probably yes, but we dont have real evidence to support it. But logic is saying that this probably happened.
