Adam Clarke on Esther 8:8

May no man reverse - Whatever had passed the royal signet could never be revoked; no succeeding edict could destroy or repeal a preceding one: but one of a similar nature to the Jews against the Persians, as that to the Persians was against the Jews, might be enacted, and thus the Jews be enabled legitimately to defend themselves; and, consequently, placed on an equal footing with their enemies.