(a) The CHASAM SOFER (Yoreh Deah #233) says that this is a Kal v'Chomer
d'Oraisa, since we are deriving the obligation to praise Hashem when He saves
us through a miracle from the Shirah that the Jewish people said when they
were saved from Mitzrayim which seemed to be a fulfillment of a Torah
obligation to sing praise. Accordingly, this would explain the opinion of the
BEHAG, quoted by the Ramban in Sefer ha'Mitzvos (Shoresh Rishon), who
maintains that Hallel (on Chanukah) and Mikra Megilah (on Purim) are Mitzvos
*d'Oraisa*. The Kal v'Chomer of our Gemara teaches that there is a Chiyuv
d'Oraisa to thank Hashem when He saves us through a miracle.
The Chasam Sofer says that this does not mean that there is a Mitzvas Aseh
d'Oraisa to light the Neros of Chanukah and to read the Megilah on Purim.
Rather, it means that mid'Oraisa, there is a requirement to make some display
of praise to Hashem to show appreciation for the miracle. The Chachamim
instituted what form of praise to make -- in the case of Purim, it is the
reading of the Megilah. However, even if one does not fulfill the rabbinical
requirement but makes any display of praise to Hashem, he fulfills the Torah
obligation to praise Hashem for the miracle.
(b) The NETZIV (Ha'Emek Sh'eilah, Vayishlach 26:1) challenges this
explanation of the Chasam Sofer. First, he asks, how can we derive an
obligation to praise Hashem every year for a miracle that occurred once, from
when the Jews praised Hashem at the Sea for the miracle that occurred then?
When they praised Hashem for taking them from slavery to freedom, that Shirah
was said *at the time* that the miracle actually took place. From there we
can only learn that there is an obligation to praise Hashem *at the time that
the miracles occurs*, but not that there is an obligation to praise Hashem
every year on the anniversary of the miracle! How can there be a Chiyuv
d'Oraisa for that?
Second, we may ask according to the Chasam Sofer, who understands that the
Gemara is saying that we learn from the Shirah at the Sea that there is an
obligation to make a display of praise to Hashem for His miracle, why does
the Gemara ask that we should also say Hallel on Purim because of this Kal
v'Chomer ("Iy Hachi, Hallel Nami Neimra?")? What is the question of the
Gemara? We make another display of praise for the miracle -- we read the
Megilah, send Shaloch Manos, give Matanos l'Evyonim, etc.! Why does the
Gemara ask that we should say Hallel, if any display of praise suffices to
fulfill the Torah obligation?
Third, the Netziv asks, according to the Chasam Sofer, how can the Gemara
(Rosh Hashanah 19b) say that the days of Yom Tov of Megilas Ta'anis were
annulled? Those days were all days on which miracles took place, and the
Chachamim established them as days of Simchah to give praise to Hashem for
His miracles. According to the Chasam Sofer, there is a Chiyuv d'Oraisa to
give praise to Hashem for His miracles, and if so, how can those days become
annulled so that *no* display of praise is made on those days? According to
the Chasam Sofer, refraining from giving praise to Hashem on those days
constitutes a transgression of a Mitzvas Aseh d'Oraisa! (RAV CHAIM ZIMMERMAN,
zt'l, in "Agra l'Yesharim," ch. 19-21, discusses this at length and points
out that the son of the Rambam already debated this point with Rav Daniel
ha'Bavli in Sefer MAASEH NISIM, #1.)
To answer these questions on the Chasam Sofer, we might suggest as follows.
The Kal v'Chomer that teaches that we must praise Hashem when He saves us
through a miracle actually includes two parts. First, there is the Kal
v'Chomer from the Shirah at the sea which obligates us to praise Hashem *at
the time* that a miracle occurs. Second, there is another Kal v'Chomer from
the annual commemoration of the miracle that the Torah requires us to observe
-- the Yamim Tovim. We are obligated by the Torah to observe the Pesach and
Sukos as a display of praise to Hashem for the miracles that He did for us
many years ago. If we have an obligation to perpetually commemorate a miracle
which brought us from slavery to freedom, then Kal v'Chomer we must make a
perpetual commemoration of a miracle which brought us from death to life!
Thus, the Kal v'Chomer teaches that we must praise Hashem for the miracle
when it happens, and also that we must commemorate it each year on the
anniversary of the miracle!
Regarding the second question, when the Gemara asks that we should say Hallel
on Purim, even though we already perform another display of praise to Hashem
for the miracle, the Gemara is asking that we should say Hallel since it
should be preferable to use the *exact* form of thanking Hashem that is
expressed in the Torah (i.e. singing a song of praise, like Hallel, to
Hashem). Why should we instead enact a new way of praising Hashem (such as
Mikra Megilah)? The Gemara answers (among other answers) that "reading the
Megilah replaces the recitation of Hallel." That is, it is certainly
appropriate to read the Sefer that was instituted to record the events of
Purim. But once we have done that, it is no longer necessary to recite
Hallel, since we have already commemorated the events by reading the Megilah.
Regarding the third question, how could the Chachamim annul the days of
Simchah of Megilas Ta'anis, it could be that those days were different than
Chanukah and Purim. The obligation to praise Hashem for those miracles is
*not* included in the Kal v'Chomer, and is thus only d'Rabanan. On those
days, the miracles that occurred were not obvious and open miracles. A
heretic could easily attribute them to natural causes and coincidences (see
Ta'anis 17b-18b).
For example, the twenty-eighth day of Adar was instituted as a day of Yom Tov
to commemorate the miracle that occurred when the Jews, led by Rebbi Yehudah
ben Shamua and his colleagues, held a demonstration to protest the harsh
decrees that the Romans had issued against them, and they successfully had
the decrees rescinded. That miracle could easily be attributed to natural
factors, such as the political pressure exerted by a prominent segment of the
population lobbying on behalf of social reform. Other days commemorated the
deaths of Roman antagonists, or successfully quelling the protests of
Tzedukim to certain Mitzvos, etc. The Chiyuv d'Oraisa to give praise to
Hashem for His miracles, as derived from the Shirah, only includes those
miracles that are similar to the redemption from Mitzrayim -- open and
obvious miracles. It does not include miracles that are not obvious. At
Chanukah and Purim, the miracles that occurred were obvious -- on Chanukah,
the few and weak Jews defeated the large and mighty army of the Syrian-
Greeks, and on Purim, an unexplainable, sudden reversal of the plans of Haman
occurred. One moment he was the highest ranking citizen in the country, the
next moment he was headed for the gallows -- when it had previously seemed
impossible to abolish his evil decrees. Therefore, those miracles are
included in the Chiyiv d'Oraisa to give praise to Hashem. (M. Kornfeld)