When thinking about the significance of Chanukah and the cause for celebration, one will think of two miracles. One, the incredible victory of the few over the many, the weak over the mighty and second, the miracle associated with finding the only sealed jar of pure oil for the Menorah that was to last a day yet miraculously lasted for eight.
One would assume that in mentioning the miracles of Chanukah we would mention both. Yet upon observation we find an interesting phenomenon that separates these two miracles completely. In the text of the Al Hanisim which we recite in the Shmone Esre and in Birkat Hamazon there is no mention of the miracle with the oil. (While there is mention of “kindled lights in Your holy courtyards”, some comment that this does not refer to The kindling of The Menorah (in the Heichal) and the miracle with the oil, but to a general kindling of many other “Menorahs” (in the “courtyards”) in celebration and thanks to Hashem.
The Talmud (Shabbat 21b) on the other hand, in relating the story of Chanukah focuses almost entirely on the miracle of finding the jar of oil and it lasting for eight days. The victory of the Jews over their oppressors is mentioned almost as a passing remark in order to set the stage for finding the jar of oil. The institution of Chanukah as days of thanks and joy is due to this miracle and not associated with the actual military victory.
One possible explanation is connected with the idea that the challenge that Am Yisroel faced during this particular period of time was not as much a physical danger and persecution as a spiritual one. The Yevanim persecuted Jews for keeping Hashem`s Torah, keeping Hashem`s Mitzvot. Not as much about the wisdom of Torah but more about it being Torat Hashem. You will note that Purim is celebrated with a feast, giving charity and sending gifts of food since it is a commemoration of rescue from a near annihilation. Chanukah on the other hand is celebrated as a salvation from a spiritual repression.
The military accomplishment and miraculous victory that defied all odds is acknowledged with our thanks and praise during the Shmone Esre & Birkat Hamazon in the blessings of thanks. However the true cause of instituting the celebration was the spiritual freedom that ensued. The nature of this miracle is signified through the miracle associated with kindling the Menorah as the Passuk (Prov. 6:23) states Ki Ner Mitzva ve`Torah Or, A mitzvah is a candle and Torah is light.
The sages did not want to mar or give room for deviation as to the real cause of joy and celebration. Thus they made a clear distinction. They offer thanks and praise for the physical victory while not mentioning anything about the miracle with the oil. On the other hand they state clearly that the true cause for Chanukah as days of joy and thanks is due to the spiritual salvation alone.