The Romans had no regular law-enforcement system. But when an important offence had been committed a group of citizens could be assembled by any magistrate to act as law-enforcement officers.
Such a group was called a Posse Comitatus (a Task Force).
Posse comitatus gives us the word Posse in English, and is as close in meaning to Band of Brothers as you are going to get in real Latin. (When Shakespeare invented the phrase Band of Brothers he intended it in a very near sense to this).
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