On the Tenth of Tevet Nebuchadnezzar King of Babylon placed Jerusalem under siege. The wicked ruler brought famine and distress upon the city, until it was finally conquered and its Holy Temple destroyed.
Fast days of this sort are not merely occasions for recalling the tragedies of our past. They are, more than anything else, days of repentance and introspection. Moreover, the Torah commands the Jewish people to cry out and to sound trumpets even when new calamities beset us, and the Sages established a Mitzvah to fast in response to such hardships, until God shows mercy upon us.
Rabbi Moshe ben Maimon, the "Rambam," describes the sort of calamities which justify such behavior: when foreign nations wage war upon the Jewish people, or impose taxes upon us, or attempt to take our land away from us, or make decrees to nullify Torah commandments - even the most seemingly insignificant among them. In such cases we are obligated to fast and to cry out to God until He shows mercy upon us.
And here, we find that in the past few years nations have indeed come upon us and demanded that we relinquish portions of the Land of Israel to strangers. This is one of the calamities concerning which we are supposed to fast and cry out about. It follows that the fast of the Tenth of Tevet, in our day, must be directed towards the rectification of this calamity. Our fasting must be an attempt to rebuff the pressure of the nations, together with their threats of terrorism and war, and their desire to take our land from us.
Therefore, let this day be a day of fasting, prayer, and crying out to the God of Israel, that He provide us with the strength to defend our land, to stand up to pressure, and to never, under any circumstances, give in to the forces which desire to destroy us.