The mitzvah of kindling the Chanukah lights begins with sunset (Shabbos 21b).
Chanukah commemorates both physical and spiritual triumphs. Israel had been politically, that is physically, under the domination of the Greek-Syrians, and the Hellenist culture was jeopardizing the spirituality of Judaism. The miracle of Chanukah, which occurred at one of the darkest moments in Jewish history, should remind us that no matter how bleak life may appear, whether in a physical or spiritual sense, we should never abandon hope. Hence, we commemorate Chanukah in the evening, when it is just beginning to get dark.
We might ask, "Why light the candles at dusk? Why not wait until it is completely dark, when the candles will shine their brightest and banish the total darkness?"
In my work with alcoholics, I often hear that "one does not recover until one hits rock bottom." However, the changes that may occur on the way to rock bottom are often so irreversible and catastrophic that rehabilitation programs put in much effort and ingenuity to intervene at an earlier stage.
We light the Chanukah candles when the sky is just beginning to get dark, instead of waiting for complete darkness. Our action teaches us when we should combat moral and spiritual deterioration - at the very first indication that it is occurring. Delaying action until the latter has occurred may be too costly.