The holiday of Chanukah is a preparation for the time of Moshiach:
מדליקיו שמונת ימי חנוכה = משיח
A hint of Chanukah can be found in the Torah in parshas Emor. After the Torah tells us about all of the Yomim Tovim, the Torah tells us ויקחו אליך שמן זית זך. Some say this is a reference to Chanukah.
The Gemara asks מאי חנוכה "What is Chanukah?" What is the Gemara’s question? Don’t we all know about the holiday of Chanukah? Rabbi Meir Schapiro answers that the Gemara means to ask, "What is the essence of Chanukah… What gives us the strength to fight the evil and our evil inclination?" The answer is the holy flames, the flames of Torah and mitzvos give us the strength to live according to Hashem’s will.
There were many miracles that happened on Chanukah, so why did the Sages establish the holiday based on the miracle of oil? Because the other miracles, like winning the war and the many in the hands of the few, etc. could sometimes be mistaken for natural occurrences that just happened to work out in favor of the Jews. With the miracle of oil it was a clear miracle and everyone had to admit that Hashem made it happen.