Twelve years into Ahaz's reign in Judah, Hoshea starts up as king in Israel. He continues doing evil religious practices—presumably the "sins of Jeroboam"—but he apparently wasn't as bad as the kings who came before him.
He reigns for nine years and becomes a vassal of the King of Assyria. However, he betrays the King of Assyria by trying to create an alliance between Israel and Egypt and failing to pay his tribute to Assyria.
The King of Assyria is outraged. He imprisons Hoshea.
The Assyrian King invades Israel and takes over Samaria. He sends the people of Israel into exile, making them live in a city called Halah in the land of the Medes.
A Brief Explanation
The narrator gives his interpretation of these events:
This disaster happened because the people of Israel abandoned God, forgetting how he'd saved them during the Exodus.
They started to worship other gods and adopt foreign customs. They worshipped at high places, using sacred poles and pillars on hills and trees like the nations that God drove out of the land earlier.
They worshipped idols and failed to listen to all the prophets, who commanded them to do the right thing and obey God's commandments.
The people also made sacred images of two calves (related to the "Sins of Jeroboam"), worshipped Baal and the host of heaven (the stars and planets), sacrificed their children in fire, and practiced divination.
So, the narrator concludes, that's why the northern kingdom of Israel was destroyed and why the people were sent away. Only the tribe of Judah remained intact (down in Judah).
However, Judah didn't follow God's ways correctly either.
The narrator places the blame on "the sins of Jeroboam." They led Israel astray and ultimately caused the exile in Assyria which lasted until the narrator's day.