Q. The Talmud asks “What is Chanukah?” Didn’t the rabbis know?A. The question is indeed asked in the Talmud in tractate Shabbat 21b (cf. Megillat Ta’anit 9). Strangely, no such question is asked about the other festivals (there is a great deal in the rabbinic sources about Purim, the other “minor” festival, yet nowhere do the sages ask, “What is Purim?”).
Nonetheless the Talmud is well aware that Chanukah is a festival of lights, since it discusses the laws of the lights in a number of places (e.g. Shabbat 22a, Bava Kamma 62b, Middot 1:6).
The discussions have a specific angle on the story: there was a flask of oil that kept on keeping on, so we remember it by kindling lights each evening. Nothing about Judah the Maccabee, no reference to Maccabean heroism. The military and political factors might well never have happened, and that is probably the reason for the “What is Chanukah” question.
The sages asked their question because they knew what answer they wanted to hear, that there had been a religious crisis – the pollution of the Temple – and there was a spiritual answer – God ensured that the sanctuary could be restored and re-opened.