Two of the mitzvos particular to Purim are Mishloach Manos, and Matanos L’Evyonim – giving gifts to people, and distributing charity freely. The Sfas Emes explains that the function of these mitzvos as they relate to Purim is that they increase unity and brotherhood.
Unity is the anathema of Amalek, who Haman was descended of. His complaint to Achashverosh:
יֶשְׁנוֹ עַם אֶחָד מְפֻזָּר וּמְפֹרָד בֵּין הָעַמִּים בְּכֹל מְדִינוֹת מַלְכוּתֶךָ וְדָתֵיהֶם שֹׁנוֹת מִכָּל עָם – There is one nation, scattered and dispersed among all the regions of your kingdom, and they are different from everyone else. (3:8)
Even in exile, Jews must maintain identity, and resist assimilation. Haman points out their refusal to integrate, they remain עַם אֶחָד – one nation; this in spite of how the Purim story begins with the Jews attending Achashverosh’s party celebrating their own downfall with the parading of the sacked Temple’s artefacts. The Jews lost their identity and it paved the way for Haman’s nefarious plans to destroy them all – the moment they let their guard down.
The resolution came at the hand of Mordechai and Esther. She tells him to unite the people and impress on them the severity of their futures:
כְּנוֹס אֶת כָּל הַיְּהוּדִים הַנִּמְצְאִים בְּשׁוּשָׁן וְצוּמוּ עָלַי וְאַל תֹּאכְלוּ וְאַל תִּשְׁתּוּ שְׁלֹשֶׁת יָמִים לַיְלָה וָיוֹם – Gather all the Jews in Shushan. Fast for me; don’t eat or drink for three days and nights. (4:16)
The threat is faced when they gather once more, when the Megila tells us that וְעָמֹד עַל נַפְשָׁם – it does not say ועמדו in the plural, that they stood for their lives, but in the singular. Their national identity had discovered. The Jewish nation had united and defended itself from attack.
It is famously expounded in Chazal that Purim also celebrates קימו מה שקיבלו כבר – the Jews had no choice to accept the Torah at Sinai, but after Purim they accepted the Torah afresh, voluntarily. A prerequisite to the Torah is unity; ויחן שם נגד ההר – The nation camped by the mountain, in the singular – not ויחנו – like one man with one heart. The Sfas Emes teaches that וְעָמֹד עַל נַפְשָׁם is directly parallel to ויחן שם נגד ההר.
Unity is fortified with acts of ואהבת לרעך כמוך – loving ones fellow as oneself. If people identify with the nation, they have a very direct connection to the Torah and Sinai. It is quite reasonable to suggest that due to this, it is taught that זה כלל גדול בתורה.
The Gemara says that Mordechai is identified as an איש יהודי. It asks that he was not from Yehuda, but from Binyamin, and answers that we do not read it יהודי, but יחידי – from the root אחד. He brought unity and identity back to Jews who had lost it, cementing their faith, culminating in a new acceptance of the Torah. All mitzvos of the day will reflect unity and friendship to some degree.
The way to fight Amalek is a constant quest for unity and understanding our identity, and the closer we get, the nearer we get ultimate truth and redemption.