Posuk yud zayin in Perek chof heh in Chumash Devarim tells us that we should remember what Amalek did to Bnei Yisroel. That is why we have the mitzvah of Parshas Zachor, to be leined for all to hear.
The Gemara in Brachos, daf mem zayin, amud bais brings down a story about Reb Elazar who came to Shul and found that there were only nine people for the minyan. Reb Elazar freed his Eved Knani, Tevi, who then completed the minyan. The Gemara questions how he was allowed to do that if Reb Yehuda says that one who frees his slave is oiver an aseh that one should keep an Eved Knani forever. The Gemara answers that since minyan is a mitzvah of a rabim, it therefore was allowed.
The Rosh in siman chof says that the mitzvah of a rabim is stronger than a regular mitzvah because the posuk says “Venikdashti” amongst Klal Yisroel, the source for a minyan. This premise will hold true even if it is for a mitzvah deRabbanan like to hear kedusha or borchu; nevertheless, it will be doche a mitzvah deOraysa of a private person of not freeing a slave. The Rosh concludes that this must be the case because the gemara is not speaking in a case where the tzibbur was laining Parshas Zachor which is min haTorah, so we see that this point of a tzibbur being stronger is even if one is just doing a mitzvah deRabbonon.
The Trumas Hadeshen, based on this Rosh, says in siman kuf ches that the mitzvah of Parshas Zachor is to listen to it with a minyan. Therefore, if one has only one of the following options of either hearing Zachor with a minyan, or Krias Hamegillah with a minyan, one should choose Zachor with the minyan since the mitzvah is min haTorah. The Shulchan Aruch in siman kuf mem vov, seif bais concurs.
The Pri Chodosh in seif koton bais asks where did the Rosh learn this halacha that one needs a minyan for the reading of Zachor? In addition, why don’t find a person has a chiyuv to hear any of the other mitzvos of zechiros (for example, remembering Yetzias Mitzrayim or Maaseh Miriam) betzibbur?
The Hagahos Peulas Sachir on the Sefer Maaseh Rav for the Gra says that the chiyuv of tzibbur for Amalek will come as a result of the shita of the Yereim in siman taf lamed heh that holds that the mitzvah of erasing Amalek is not a mitzvah for every private person, but rather a mitzvah on the tzibbur under a king. With this in mind, it makes sense that we should read the parsha of Amalek betzibbur to remind us that this is a tzibburdige din.
There are many poskim who disagree and say you do not need a minyan; however, according to the Rosh we can understand why we need a minyan.
Let us hope we are zocheh to the tzibbur and the king Moshiach to wipe out Amalek!
