Why was the Northern Kingdom of Israel destroyed by the Assyrians?

Answer 1

II Ki 17:7-23 "For so it was that the children of Israel had sinned against the LORD their God, who had brought them up out of the land of Egypt, from under the hand of Pharaoh king of Egypt; and they had feared other gods, and had walked in the statutes of the nations whom the LORD had cast out from before the children of Israel, and of the kings of Israel, which they had made.

Also the children of Israel secretly did against the LORD their God things that were not right, and they built for themselves high places in all their cities, from watchtower to fortified city. They set up for themselves sacred pillars and wooden images on every high hill and under every green tree. There they burned incense on all the high places, like the nations whom the LORD had carried away before them; and they did wicked things to provoke the LORD to anger, for they served idols, of which the LORD had said to them, "You shall not do this thing."

Yet the LORD testified against Israel and against Judah, by all of His prophets, every seer, saying, "Turn from your evil ways, and keep My commandments and My statutes, according to all the law which I commanded your fathers, and which I sent to you by My servants the prophets." Nevertheless they would not hear, but stiffened their necks, like the necks of their fathers, who did not believe in the LORD their God. And they rejected His statutes and His covenant that He had made with their fathers, and His testimonies which He had testified against them; they followed idols, became idolaters, and went after the nations who were all around them, concerning whom the LORD had charged them that they should not do like them.

So they left all the commandments of the LORD their God, made for themselves a molded image and two calves, made a wooden image and worshiped all the host of heaven, and served Baal. And they caused their sons and daughters to pass through the fire, practiced witchcraft and soothsaying, and sold themselves to do evil in the sight of the LORD, to provoke Him to anger.

Therefore the LORD was very angry with Israel, and removed them from His sight; there was none left but the tribe of Judah alone. . And the LORD rejected all the descendants of Israel, afflicted them, and delivered them into the hand of plunderers, until He had cast them from His sight. For He tore Israel from the house of David, and they made Jeroboam the son of Nebat king. Then Jeroboam drove Israel from following the LORD, and made them commit a great sin. For the children of Israel walked in all the sins of Jeroboam which he did; they did not depart from them, until the LORD removed Israel out of His sight, as He had said by all His servants the prophets. So Israel was carried away from their own land to Assyria, as it is to this day."

Answer 2

1) The spiritual reason was that for over two centuries the Ten Tribes had cut themselves off from the more-righteous tribe of Judah and the Temple, and had been more lax about the prohibition against idolatry. The prophets, such as Hosea, had warned them but were not heeded widely enough.

2) The temporal reason was that Assyria would conquer whomever they could (like many other ancient nations).

(See also: Where are the Ten Tribes?)

A positive result: Had the Ten Tribes not been exiled, the Tribe of Judah would have continued emulating their ways (Jeremiah 3:7-8) and might have gone lost like the Ten Tribes.

The First Destruction (some 2500 years ago):
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