The full sentence is, of course,
Aio te, Æacida, Romanos vincere posse.
This is the (literally) classic example of a prophecy that can be taken however
the hearer chooses.
It may mean either
I tell you, Æacida (Pyrrhus), that you can conquer the Romans.
I tell you, Æacida (Pyrrhus), that the Romans can conquer you.
Ennius reports this as a reply given Pyrrhus by the Delphic Oracle.