Chanukah began as the celebration of the rededication of the Temple after the Maccabean Revolt. In the early days it was a celebration of the military victory that established the rule of the priestly Hasmonean Dynasty. The Maccabees threw off the Greek ruler with military prowess, and celebrated by rededicating the Temple with a festival to replace the festival of Sukkot which the Greeks had made impossible that year. The Jews continued to celebrate it for eight days beginning on 25 Kislev every year, and they called it the Festival of Lights. We know this from a book by Josephus, who wrote about it about 250 years later.
The next we hear of the holiday, it is mentioned in passing a few times in the Mishnah, 200 years later. (for example, M. Bava Kama 6:6) but one gets the distinct idea that the rabbis don’t like to talk about it. Also, it has changed names: now it is Chanukah [Dedication.] When the rabbis finally do talk about it in the Gemara, a few hundred years after that, it has become a holiday based on the miracle story of a single bottle of oil that lasted for eight days.
Why the change? Why no mention of the military festival for several hundred years, and then this miracle story? In the meantime the Jewish People had had two great disasters, both associated with attempts to throw off the Romans with an armed uprising. The disaster was the destruction of the Temple in the year 70. The second disaster was the failure of the Bar Kokhba Revolt in 136. As Rabbi Lawrence Schiffmann wrote, “By the end of the [Bar Kokhba] war many Jews had been massacred, the land had been devastated again, and distinguished rabbis had been martyred.”
So it is no wonder that the rabbis did not encourage the celebration of the old Festival of Lights. It celebrated a military uprising, and subsequent uprisings were disasters. They turned instead to the miracle story of the oil, to turn young eyes from the glitter of weapons to the peaceful glow of the menorah in a dark night. That is also why you will hear people insist, “It’s a minor holiday.” There is a tradition for playing down Chanukah.