The Disappearing Ninth Legion
The Roman army’s Ninth Legion successfully conquered England in A.D. 43 and—with the exception of the odd rebellion—kept control of the bottom half of Britain for the next 74 years. Then, from A.D. 117 onward, the entire legion suddenly vanished from the historical record. Today, no one has any idea what happened to them.
One of the most popular theories is that they marched north into Scotland to fight an uprising of the Picts. This story has gained traction in fiction as the basis of several books and films, but some historians believe the legion simply left to go fight elsewhere. It’s also been speculated they were defeated in a battle against Rome’s nemesis, the Parthians, in Iran. Another possibility is that they lost against a Jewish uprising in A.D. 132.
One clue suggests that things had started to go wrong for the Ninth even before that time. When Emperor Hadrian turned up in A.D. 122, he brought another legion with him, the Sixth. He proceeded to build a wall across the north of England, then called Britannia, to keep out the people north of the border. If they had recently wiped out a renowned arm of his military, that would have beeen a very good reason to erect such a structure.