The Jews who live in Eretz Yisrael have often been characterized as suffering from a "siege mentality" which causes them to view the world from a paranoiac perspective of "they are all against us."
"Paranoids", a statesman once said, "also have real enemies."
And siege, it may be added, is not a matter of mentality but the "real thing" repeated throughout our history.
The first siege of Jerusalem took place more than two and a half millennia ago when the Assyrian world conqueror, Sancherev, threatened to invade this last bastion of Jewish resistance. The unparalleled proliferation of Torah study promoted by the righteous King Chizkiyahu won for him the miracle of an angel slaughtering the entire enemy force.
But the spiritual degeneration, which followed Chizkiyahus reign eventually, led to another siege. On the tenth day of the month of Tevet the Babylonians, under the command of Nevuzadran, laid siege to Jerusalem and it was only a matter of time until they penetrated the city walls, destroyed the Beit Hamikdash, and took the people into exile.
When Jews fast on the day that this siege began they are expected to internalize the lesson that history teaches us. When Jews break down the walls of discipline, which the Torah has erected for them, they expose themselves to a Heavenly response of a real siege from real enemies threatening to penetrate their walls.