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... mero debellata, monet Sithoniis non levis Euhius, ...

Sithoniis non levis: dat. with "levis," "not gentle/mild to the Sithonians" (a Thracian tribe reputed to over indulge in alcohol); for the meaning and syntax, cf. OLD 11b s.v. "levis," citing this very phrase. Note the difference between "levis" w. short "e" meaning "light, nimble, insignificant, gentle" and "levis," w. a long "e" meaning "smooth."