Once “those people” control your military, they control your country.
THAT the Romans discovered happened with Goths and other Germanic troops recruited to serve in the 300’s and 400’s.
According to Romano-British Chronicles, the Romano-Britons recruited Anglo-Saxon mercenaries to fight for them against the Picts and Scots. I think you know the rest of the story.
The rest of the story is that as the Roman Empire was having trouble in the 400s much closer to home, Britain was essentially written off and abandoned to fend for itself .