"V'Choshech al pnei tehom" -- We all learned in history books about the Dark Ages between 500-1000, but Midrash darshens that there was a different dark ages. "Choshech," explains the Midrash, is the time of the Greek empire. Even though there was culture, philosophy, art, etc. in Greece, it was a time of great spiritual darkness.
We unfortunately have been in a few galiyos -- Bavel, Paras, Edom, and Greece. Couldn't each galus be rightfully described as a time of spiritual darkness? Why is the Greek empire in particular singled out?
Sefas Emes answers that in each of the other galiyos we lacked the Beis haMikdash. It's no big deal to say that galus caused darkness when the greatest source of light in the world was absent. The tragedy of the Greek oppression is that it happened when we had a Beis haMikdash, the oro shel olam. To bring darkness to the world when there is that bright light that should be shining -- that's something. Because we were able to overcome Yavan, we celebrate the Festival of Lights. Because they tried to extinguish the light, in the end there is more light brought into the world.
I think I once posted the Sefas Emes' question: Moshe went up to Sinai for 40 days to get the Torah, he spent 40 days there davening for Hashem to forgive the cheit ha'eigel. Why did he need to spend another 40 days there to get the luchos sheniyos and get the Torah a second time? He knew it all from the first 40 days -- it's the same Torah laws?
Sefas Emes answers that b'makom she'ba'alei teshuvah omdim afilu tzadikim gedolim einam yecholim la'amod. After the middle 40 days Bnei Yisrael were on the level of ba'alei teshuvah as they had repented from the cheit ha'eigel. The Torah of the ba'al teshuvah is a different Torah, a higher Torah. It took another 40 days to learn that Torah.
Chanukah is the chanukas hamikdash, the rededication of the Temple. But the Mikdash had not fallen, it had not been dismantled or destroyed. Why did it need to be rededicated?
Here too, explains the Sefas Emes, Bnei Yisrael had sinned and become corrupted by Greek ideology. Chanukah was a time of teshuvah. Bnei Yisrael rejected that ideology and returned to Torah. B'makom she'ba'alei teshuvah omdim afilu tzadikim gedolim einam yecholim la'amod. The Beis HaMikdash post-teshuvah is a different Beis haMikdash, a higher, more exalted Beis haMikdash, than the Beis haMikdash which had always been there from beforehand. That Mikdash needed to be dedicated.
The Rambam writes that Chanukah marks the restoration of the sovereignty of the Jewish people in Eretz Yisrael, at least temporarily. We cannot recite hallel on Purim because "akatei avdei Achashveirsh anan," we are still ruled by Achashveirosh, but we do recite full hallel on Chanukah. Sefas Emes elsewhere writes that the chanukas hamikdash was really a dedication of the mikdash of the future geulah, when we will once again rule Eretz Yisrael independently, have a Mikdash, and be immune from outside influences. Chanukah is a taste of the future.
We have a Yom Tov derabbanan, writes Sefas Emes, that corresponds with each one of the Yamim Yovim, the three regalim, of the Torah. On Shavuos we received the Torah, and so we have a holiday of Purim where "kiymu v'kiblu," we affirm that acceptance. We have Sukkos where we offer korbanos that correspond to the 70 nations, and on Chanukah, we light counting down according to Beis Shamai, to correspond to those korbanos (there are other links as well). Pesach is the holiday of redemption, and in the future we will one day have a holiday of redemption as well, a holiday to celebrate our freedom and our return to Eretz Yisrael (check your calendar in a little over four months : ).