Every Chanukah we celebrate the remarkable victory of the Jewish people over the mighty Greek army, and the subsequent miracle of the single flask of oil that lasted eight days. We celebrate these events by lighting a Menorah for eight days and by saying the 'Al HaNissim' prayer and Hallel. The Sifsei Chaim notes that there is a lack of clarity as to exactly which aspect of the Chanakah story is the most significant - that of the military victory or that of the oil:
On the one hand, the Al HaNissim tefilla mainly makes mention of the defeat of the Greeks; it stresses the miraculous nature in which Hashem enabled the Hasmoneans to emerge victorious. "And You in your great mercy, stood by them in their time of distress, You defended their cause, You judged their grievances, You avenged their vengeance. You delivered the mighty into the hands of the weak, the many into the hands of the few, the defiled people into the hands of the undefiled, the wicked into the hands of the righteous, and the insolent [sinners] into the hands of the students of Your torah..." A brief reference is made of the fact that the Hashmoneans kindled the lights in the Beis HaMikdosh and no mention at all is made of the actual miracle of the oil lasting eight days!
In contrast, the gemara places a much greater emphasis on the miracle of the oil than the military victory. The gemara asks, "What is Chanukah?" It answers with a braisa that stresses the miracle of the oil and only makes a fleeting reference to the battle. "On the 25th of Kislev, there are eight days of Chanukah on which one may not eulogize or fast. For when the Greeks entered the sanctuary, they defiled all the oils in the sanctuary and when the Kingdom of the Hasmonean became stronger and overcame them, they searched and could only find one flask of oil that had the seal of the Kohen Gadol. It only had [enough oil] to last for one day, but a miracle took place and they lit from it for eight days. The following year, they fixed these days and made them festive days of praise and thanks."
How can we understand the seeming contradiction as to what was the most important miracle in the Chanukah story? In order to answer this, it is necessary to develop our understanding of miracles. It seems that there are two factors that define the significance of a miracle. One is the necessity of the miracle - the greater the urgency of the situation that led to the miracle, the more important the miracle. For example, the miracle of the splitting of the sea is an extremely important miracle in that it saved the Jewish people from a seemingly desperate situation. However, there is another aspect that helps define the significance of a miracle - that is the extent to which the miracle clearly broke the regular laws of nature. We understand that all of nature is, in effect, 'miraculous', in that it is guided by Hashem's hand. 'Nature' is merely Hashem's mechanism for keeping the world going. It takes no effort for Hashem to break these laws of nature, however in His wisdom, He rarely chooses to do so. This is because open miracles take away the free will of a person in that they make it far it more difficult for him to justify his behavior when it is contrary to Hashem's will. Accordingly, on the rare occasions that He does break the laws of nature, there is a powerful effect on the people who witness the miracle, as there is n way for them to ignore the clear act of Divine Providence. Thus, the degree to which a miracle breaks the laws of nature also plays a key role in defining its significance.
It seems that the miracles of the military victory were more important than the miracle of the oil in one of these factors, and the miracle of the oil was more important in the other. In terms of necessity, the miraculous victory over the Greeks was more vital than that of the oil. The Greek decrees against Torah observance were making it impossible for Torah Judaism to continue. Thus, it was essential that the small Jewish army overcome the mighty Greeks. However, the miracles that enabled this victory to take place were not 'open' miracles, in that they did not overturn the regular laws of nature. Accordingly, it would be possible for an onlooker to ascribe the victory to the superior military prowess of the Hasmoneans or to sheer 'good luck'.
In contrast, the miracle of the oil was not of the greatest necessity - without it, the Jewish people would still be free of the Greek yoke. However, the miracle was remarkable in that it represented a clear overturning of the laws of nature. Such a miracle had a particularly powerful effect on the onlookers, in that it made clear Hashem's involvement in an unmistakable fashion.
With this understanding we can now answer why the gemara focuses on the miracle of the oil, whilst the Al HaNissim tefilla emphasizes the victory over the Greeks . When the gemara asked, "What is Chanukah", Rashi explains that it was asking, "for what miracle did they fix Chanukah [as a permanent festival]." The Sifsei Chaim explains that, initially there were numerous events in which miracles took place, and that each one was made into a kind of Yom Tov where it was forbidden to eulogize and fast However, these events became so abundant that the Rabbis cancelled all these days of celebration with two exceptions - Purim and Chanukah. The Sifsei Chaim explains that the miracles that occurred on these days were the ones that most effected the people. In this vein, he writes that the most outstanding miracle on Chanukah was that of the oil, not of the military victory. Thus, when the gemara asked for which miracle did they fix Chanukah, it was asking which miracle was so outstanding that the Rabbis did not annul the festival of Chanukah in the way that they did almost all of the other festivals. Accordingly, the gemara answered by focusing on the miracle of the oils because that was the miracle that broke the laws of nature and therefore had the greatest effect on the people.
However, when we come to show gratitude to Hashem for the miracles of Chanukah, our main focus is on the most vital miracles, which were those that enabled the Jews to defeat the Greeks. The Al HaNissim tefilla is a prayer of thanks, therefore, the main emphasis is on the military victory, because that is the aspect of the Chanukah story that was of the utmost necessity.
The Sifsei Chaim suggests that the two concepts of Hallel and Hodaah correspond to the two different miracles. The Hallel commemorates the miracle of the oil, whilst the hodaah relates to the military victory. It is possible to add that Hallel, (ie.praise) is more apt for the oil because it showed the most outright demonstration of Hashem's involvement with the Jewish people. Whereas, hodaah is more appropriate with regard to the military victory because our greatest sense of appreciation is for the redemption from the Greek exile.
There are numerous lessons that can be learned from the Sifsei Chaim's differentiation between the two types of miracles. One key lesson he mentions is that through contemplating the open miracle of the oil we can come to a great recognition that all the other events of Chanukah, and, by extension, the other events that happen in our lives, were not chance events, but all were guided by Hashem. This increased recognition of Hashem's hand should bring us to a greater appreciation of Him. Moreover, the Alter of Kelm notes that it is not enough to feel gratitude to Hashem, rather one must also use this gratitude to bring him to a greater sense of obligation in his Avodas Hashem.