“Kol Yisrael Areivim zeh lazeh”, all Jews are guarantors for one another. While we may not always live up to this ideal, the non-Jewish world understands this principle all too well, attributing the actions of one Jew to all Jews. If Mordechai will not bow down to Haman then all Jews are to be killed. While this may be unfair it is part and parcel of the covenant G-d made with the nation of Israel. It is this oneness of the Jewish people that stands behind the concept of Kiddush Hashem or G-d forbid a Chillul Hashem.
Purim celebrates this unity of the Jewish people. “Go gather all the Jews” is the first step Esther takes as she plans to ask Achasverosh to rescind the decree against the Jews. Purim is celebrated with misloch manot, portions given to one another and matanot laevyonim, gifts to the poor. Our Sages understanding of the Megillah as a reacceptance of the Torah speaks to this unity of the Jewish people. It was at Sinai where we were encamped “with one heart like one person.”
And yet Purim is the one holiday where Jews around the world celebrate it on different days, those in open cities on the 14th of Adar and those living in walled cities on the 15th of Adar. That seems a strange way to celebrate Jewish unity. Lest one argue that such is merely a reflection of the story itself - the Jews outside of Shushan rested on the 14th of Adar whereas those of Shushan did not rest until the 15th - such appears irrelevant.
“Mordecai wrote these things, and sent letters unto all the Jews who were in all the provinces of the king Achasverosh, both near and far, to enjoin them that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly...and the Jews took upon themselves to do as they had begun, and as Mordechai had written unto them” (Esther 9:21:22).
When Mordechai wrote to the Jewish people near and far declaring Purim to be a permanent holiday he ordained that Jews everywhere should celebrate both the 14th and the 15th day of Adar as Purim. Does it make sense that Jews around the world celebrate while those in Shushan are still fighting? Shouldn’t all Jews celebrate the victory in Shushan - the place where the story took place?
And the two-day holiday for all was reaffirmed in a second letter sentthis time both by Esther and Mordechai. “Then Esther the queen, the daughter of Abihail, and Mordecai the Jew, wrote down all the acts of power, to confirm this second letter of Purim. And he sent letters unto all the Jews, to the hundred twenty and seven provinces of the kingdom of Achasverosh, with words of peace and truth, to confirm these days of Purim” (9:29-31).
Why and how this unified holiday was split into two different celebrations is not at all clear and seems to reflect a later day modification of the holiday. Rav Yoel Bin Nun suggests that this resulted from the refusal of the Jews living in Israel to accept that Jews living outside the land of Israel should be the arbiters of Jewish life. It is from Zion that Torah must emanate.
On the backdrop of the period this is quite understandable. Cyrus the Great allowed the Jews to return the land of Israel. Unfortunately very few of them actually did. So while the Jews of Shushan took up battle on behalf of the Jewish people they had failed to help build up the land of Israel. Their efforts might be worth celebrating but on terms and dates as determined in Israel. To emphasize the primacy of the land of Israel the definition of a walled city was determined to be not from the time period of Shushan (as Rabbi Yehoshua ben Korcha logically argued, Megillah 2b) but from the times of Yehoshua bin Nun, from the time the Jewish people first settled the land. The centrality of Israel lay behind the Talmudic explanation (most interestingly given to Rav Yehoshua ben Korcha) as to why Hallel is not recited on Purim; “because we do not say Hallel on a miracle outside the land [of Israel]” (Megillah 14a).
Ironically the only city which we know for certain that had a wall at the time of Yehoshua is Jerusalem, which during the miracle of Purim lay in ruins without a wall. By insisting on a “separate” celebration for the land of Israel our Sages actually linked Jerusalem and Shushan. In fact while Shushan was not walled during the time of Yehoshua Purim was still to be celebrated there on the 15th of Adar “since the miracle happened there”. Try as we may to think otherwise, the fate of the Jewish people worldwide is linked as one; what happens in one place impacts on Jews everywhere. That is a message that must “be remembered and kept throughout every generation” (Esther 9:28).
 While Jews in the diaspora have a second day of Yom Tov that reflects the doubt as to the actual day it might have been during the time when the calendar was not fixed.
 This would not be the last time that the leaders in Israel and the Diaspora would argue as to who has supremacy. It is worth noting that the “division” of Purim is the opening theme of Masechet Megillah of the Talmud Bavli considered more authoritative than the Talmud Yerushalmi.