Meaning of ON at the end of a sentence
It is the adverb (more usually "onwards" or "onward") indicating the direction in time from that specified in the sentence (403) or context (the restoration of [Athenian] democracy).
https://dictionary.cambridge.org/dictionary/english/onwards
The sentence would be understood with "on" omitted, but the usage implies a continuing danger here, as opposed to a brief period during the restoration year/s.
https://sentence.yourdictionary.com/onwards