Why did Nebuchadnezzar conquer Judah in 586 BCE and force the Jews into exile?

Answer 1

1) The general spiritual reason was that God found the people of Judah to be below the spiritual level that was a requisite for having His presence among them. The prophets had warned them (Jeremiah 7:25) but were not sufficiently heeded (2 Chronicles 36:16). Once God's presence no longer felt welcome in the Holy Temple, its destruction was just a matter of time (Talmud, Rosh Hashanah 31a, and midrash Eichah Rabbah 1:43).
A more specific spiritual reason was the egregious sins of Menasheh, king of Judah (2 Kings 21:11-13 and 24:3).

2) The temporal circumstance was the fact that King Yehoiakim, after having been obedient to Nevuchadnezzar (king of Babylonia), became insubordinate (2 Kings 24:1); and Nevuchadnezzar responded by conquering the land of Judah, destroying the First Temple and exiling the populace.

Some positive results: the false prophets, at long last, were silenced forever. They had predicted that Judah would remain independent of Babylonia (Jeremiah ch.27) and no Destruction would take place.
Also, the lure of idolatry finally weakened, since the Destruction and Exile happened exactly as predicted by the true Prophets, who were the same ones who had spoken ceaselessly against dabbling in idolatry.

The Second Destruction (in 68 CE):
1) The spiritual reason was that the Jewish people were plagued by an internal enemy, the Sadducees, who had been dragging the spiritual level of the people downwards for many decades. These were men of power who (along the lines of the earlier Hellenizers) were less interested in Torah than in pleasure, politics, and obsequiousness to the Herodian kings and the Romans. Even the once-pious Hasmoneans had eventually become infiltrated by the Sadducees. They had fomented unnecessary wars, sown discord among the Jewish people (see Talmud, Yoma 9b), and had even killed a number of the leading Torah-sages (Talmud, Kidushin 66a).

2) The political trigger to the Second Destruction was the anti-Roman revolt fomented by the Zealots, who acted against the advice of the Rabbis and left the Romans no choice but to crush the people (Talmud, Gittin 56).

A positive result: The Second Destruction, as painful and tragic as it was, at least accomplished the disappearance of the impious cliques (as alluded in the parable in the Talmud, Gittin 56b, in which the "barrel of honey" represents Jerusalem and the Temple, and the "serpent" hints to the Zealots and Sadducees, among others). Once Jerusalem and the Temple were razed, the men of power melted away and the internal life of the Jewish communities returned to the aegis of the Torah-sages. (See: What did Rabbi Yochanan ben Zakkai do?)

Other non-traditional groups including the Samaritans and the Essenes also now permanently dissociated themselves from the Jewish people.

See also:

Jewish history timeline

The Diaspora

Answer 2

because he was the boss and god told him to