The Gemara in Megillah (14a) wonders why we don't recite Hallel on Purim, and offers several explanation. Among them we find:
1. Because we don't say Hallel for miracles that took place outside of the Land of Israel
2. The reading of the Megillah constitutes a form of praise, obviating the need to recite Hallel.
Rava adds a different perspective.
According to Rava, the fact that God saved the entire Jewish nation from annihilation on Purim is not sufficient justification to recite Hallel, because they remained the subjects of the King Persia afterwards as well. As great as the salvation was, as long as the Jews remained subject to the whim of a foreign ruler, they should not recite Hallel.רבא אמר בשלמא התם הללו עבדי ה' ולא עבדי פרעה, אלא הכא הללו עבדי ה' ולא עבדי אחשורוש? אכתי עבדי אחשורוש אנןSaid Rava, Regarding [the Exodus from Egypt when the nation did recite Hallel], we can say that "these [the Jewish people] are the servants of God and not servants of Pharaoh. Yet here [in the Purim story], can we say that "these are the servants of God and not the servants of Achashverosh?" We are still the servants of Achashverosh.
We can derive, of course, the opposite rule as well. Living in an era that witnessed the rebirth of the Jewish nation, we can proudly proclaim that we no longer live as "subjects of Achashverosh." Whether we like the government's decision or dislike it - it remains a Jewish decision, made by the Jewish nation.
For that fact alone, we must declare thanks and praise to the Creator.