The people fasted for a number of days, however no rain fell. Then Rabbi Tanchuma went and spoke to them. He said, “My children, fill yourselves with compassion for one another, and the Holy One, blessed be He, will be filled with compassion for you.”
"The day of rainfall is as great as the day on which heaven and earth were created." The sages liken rain to the creation of the world: Just as the expression "gevura," might, is used in relation to Creation, so is it used in relation to rainfall.
After the destruction of the First Temple, the Prophets and the Sages of Israel legislated fasting on the Tenth of Tevet, for it was on that day that Nebuchadnezzar, the King of Babylon, and his legions placed Jerusalem under siege.