II Kings - Chapter 17
Lost: Ten Tribes
Hoshea, the last king of Israel (the Northern Kingdom of the Ten Tribes) reigned for nine years. He was conquered by Assyria and Israel became a vassal state.
Israel paid Assyria a yearly tribute. One year, Hoshea did not pay his tribute. He also tried to get Egypt to help him fight off the Assyrians. Shalmaneser, then king of Assyria, found out and he didn't care much for Hoshea's insubordination. Assyria started a three-year siege of Israel.
Assyria finally captured Samaria, the capital of Israel. The people were exiled and resettled in other lands.
That was it for the Ten Tribes. They would not be swayed from their idolatrous ways, even though G-d had corrected them many times. He sent many prophets, but the people persisted. So, G-d permitted them to be exiled and only the kingdom of Judah (including Benjamin and Levi) remained. Judah wasn't perfect, but the kingdom of Israel had pushed away from G-d with both hands. (The Navi describes this in some detail, but that's the gist of it.)
So, the Jews of Israel were exiled and the conquered people of other nations were moved into Samaria. G-d sent lions to attack the people in Samaria. When the king of Assyria found out, he had a kohein (priest) brought to Samaria to convert the people there to Judaism. They started to worship G-d, but they also continued to worship their old idols. They continued to serve G-d, but not sincerely. One of the major populations in Samaria was originally from a place called Kusa. These people are the Kusim (Cutheans) whose status as Jews is frequently discussed in the Talmud. (Let's just say they get partial credit.)