Musar sobre Ester 9:16

You could counter that when the Jews left Egypt they found themselves beholden only to G–d and to no other political power. They therefore had every reason to accept the Torah without any mental reservation! They were not in exile as were the Jews of Persia even after Mordechai's becoming Prime Minister. In fact the very fact that we do not recite the Hallel prayer on Purim is proof that we remained in exile. The remarkable thing about the political reversal during the Purim episode was that although the Jews were subjects of a foreign nation, G–d demonstrated that He did not forsake His people even in exile, and that even the mightiest king who ruled over the greatest empire was forced against his will to appoint Mordechai and reverse the decree against the Jews he himself had signed. While Ahasverus was worried about how many more antisemites the Jews had killed in various provinces of his Empire (Esther 9,16), he was forced to acceed to Esther's request to allow the Jews of Shushan to seek out their enemies and kill them on the morrow of the 13th of Adar.