[W]hen a President says “Never again” he is asking for the kind of outcome for which proving a negative, lots of negatives, is categorically essential. Proving a negative requires omniscience. Omniscience requires god-like powers.
— Dan Geer
(The whole speech is worth reading.)
Perhaps of interest, the phrase “Never Again” is used by Jews in connection with remembering the holocaust. This Stack Exchange answer provides a good summary.
http://english.stackexchange.com/a/39173
If it were claimed that the price of never having another Holocaust was the permanent imposition of a police state of the sort the Nazis had and were aiming for (after all, “proving a negative requires omniscience”), would you think it too high a price?
That’s a tough question.