Most disastrous Roman Military Defeats

[Sardaukar]

Yeah, Germanicus was the man, all right.

Likewise with Parthia's win at Carrhae. They had to make some diplomatic concessions, like returning the eagles of Crassus' legions, but they never had to deal with the legions again to my knowledge. Though Caesar would tried, had he lived.

Well, yes they had to. There was Marcus Antonius, invading in 36(?) BCE (it would even have succeeded but he lost his baggage and sienge engines due to defeat of his subordinate. His composition of his army clearly suggested he had learned from Carrhae. Parthians wisely avoided combat with his main forces.

And of course there was Trajanus who invaded successfully.