Why did hanukkah begin?

Answer 1

The Seleucids (Syrian-Greeks) under Antiochus Epiphanes (2nd century BCE), at the instigation of the Hellenizers, had forbidden various Torah-practices in Judea, such as Sabbath-observance and circumcision, rededicated the Temple to a Greek idol, and pressed the Jews to offer up idolatrous sacrifices.

Despite being heavily outnumbered, the Hasmoneans (a family of religious Jews) fought to retake the Holy Temple, which had been seized by the Seleucids, and to enable the people to once again observe the Torah. The Hasmoneans (also called Maccabees) were miraculously victorious. When they reached the Temple grounds, they immediately reconsecrated it to God. As part of this action, they relit the Menorah (Exodus ch.25), which was fueled with olive oil. However, it soon became apparent that there was only sufficient oil to keep the candelabra burning for one single day - and it would take eight days to make and bring some more.
Miraculously, the menorah stayed lit for eight days (Talmud, Shabbat 21b), allowing enough time for new oil to be prepared and brought.
The significance of the miracle is that it demonstrated that God's presence still dwelt in the Holy Temple. This is what Hanukkah represents: the closeness to God; and the avoidance of Hellenization (assimilation).
The Torah Sages instituted the festival of Hanukkah at that time (Talmud, Shabbat 21b), to publicize the miracle (Rashi commentary, ibid). This is why we light our Hanukkah-menorahs.
(The Hanukkah-menorah, or hanukkiyah, is a special form of the original seven-branched menorah. Our Hanukkah-menorahs have eight spaces for oil, or candles, to mark each of the eight days for which the oil lasted. A ninth candle nearby is called the shamash and is used to light the others.)
The eight-day rededication of the Temple is also mentioned in the book of Maccabees (I, 4:56-59; and II, 1:18); and Josephus mentions the eight-day festival in Antiquities ch.12.
The Al-Hanisim prayer which we recite during Hanukkah centers around the Hasmoneans' victory and rededication of the Temple, while the candle-lighting commemorates the miracle of the oil.
Though the military victory is prominently mentioned in the prayers, it wouldn't have been celebrated if not for the miracle of the oil, just as we have no special occasion to mark Abraham's victory (Genesis ch.14), or those of Moses (Numbers ch.21), Joshua, Deborah (Judges ch.4), Gideon (Judges ch.6-7), Jephthah (Judges ch.11), or King David. And though the Hasmonean battles continued for two decades after the retaking of the Temple, the Sages instituted Hanukkah immediately after the miracle of the oil.
It should also be noted that the main goal for which the Maccabees fought was not political independence. They fought to enable the people to observe the Torah's commandments; as we say in the Al Hanisim prayer: "the Greeks sought to cause us to forget Your Torah and leave Your statutes."

Answer 2

The Seleucids (Syrian-Greeks) under Antiochus Epiphanes (2nd century BCE), forbade various Torah-practices such as Sabbath-observance and circumcision, rededicated the Jewish Temple to a Greek idol, and pressed the Jews to offer up sacrifices to the idol. One of the leading elder Jewish sages called upon the people to keep observing the Torah anyway; and if necessary, to use force in resisting the decrees.
This brought a violent reaction from the Greeks; and the loyal Jews, led by the Hasmonean family, were forced to strike out at the Greeks in an attempt to oust them from the Holy Land and to enable the people to once again observe the Torah. The religious Jews were heavily outnumbered by the attacking Greek armies, but God gave them miraculous victories again and again.

Eventually the Hasmoneans (also called Maccabees) were able to reenter the Holy Temple which the Greeks had defiled, reconsecrated it to God, and began the Temple service once more.
Among other things, they wanted to relight the olive oil candelabrum (Exodus ch.25), but could only find one day's supply of undefiled oil - and it would take eight days to make and bring some more. Miraculously, the menorah stayed lit for eight days (Talmud, Shabbat 21b), allowing enough time for new oil to be prepared and brought.

The significance of the miracle is that it demonstrated that God's presence was still there. The Torah-community was overjoyed, because God's presence meant everything to them. This is what Hanukkah represents: the closeness to God; and the avoidance of Hellenization (assimilation).

The Torah-Sages instituted the festival of Hanukkah at that time (Talmud, Shabbat 21b), to publicize the miracle (Rashi commentary, ibid). This is why we light our Hanukkah-menorahs.

The Al-Hanisim prayer which we recite during Hanukkah centers around the Hasmoneans' victory and rededication of the Temple, while the candle-lighting commemorates the miracle of the oil. Though the military victory is prominently mentioned in the prayers, it wouldn't have been celebrated if not for the miracle of the oil.

See also the Related Links.

Link: Jewish history timeline

Link: More about the Hellenized Jews

Link: How is Hanukkah celebrated