Historically, Chanukah commemorates the victory of the Macabbees, a small group of pious Jews, who defeated the invading Syrian army. Yet, the Sages deliberately emphasized the miracle of the oil, instead of the military might of the Macabbees. Apropos of reframing the emphasis of the holiday, the Sages brought forth this pasuk (verse) as a reminder of the help the Jewish people received from G-d, when defending themselves against an army much greater than them: “Not by might, nor by power, but by My spirit, saith the L-RD of hosts” (Zechariah 4:6, JPS 1917 Tanach).
The miracle of the oil, has to do with the pure olive oil that was used in the Temple to light the seven-candled menorah that rested in the sanctuary. After cleaning up the Temple, that had been ransacked by the Syrian army, only one cruze of this pure oil was found. Regular olive oil could not be used for such a holy purpose as lighting this menorah inside of the sanctuary. Because the cruze of oil was only enough for one day, there would not have been enough time to prepare more oil, to keep the menorah burning on successive days. Yet, a miracle occurred: the single cruze of oil lasted for eight days. That is the reason we light candles for eight days on Chanukah.