After a siege of about two years, Babylonian forces under the command of king Nebuchadnezzar capture Jerusalem. Nebuchadnezzar had unsuccessfully invaded Egypt four years earlier, in 601 BCE, which made him look weak.
As a result many smaller kingdoms and city-states in the Levant rebelled against Babylonian rule. This included the kingdom of Judah with its capitol Jerusalem.
King Jehoiakim stopped paying the required tributes to Babylonian king Nebuchadnezzar and allied himself with Egypt. Despite the loss to the Egyptians, Nebuchadnezzar was not weak -- certainly not in comparison to Judah.
So he proceeded to deal with the rebellious vassals in the Levant and eliminated all of those who challenged his rule.
The Babylonian Chronicles describe events thus:
"In the seventh year (of Nebuchadnezzar-599 BC.) in the month Chislev (Nov/Dec) the king of Babylon assembled his army, and after he had invaded the land of Hatti (Syria/Palestine) he laid siege to the city of Judah. On the second day of the month of Adar (16 March) he conquered the city and took the king (Jeconiah) prisoner.
He installed in his place a king (Zedekiah) of his own choice, and after he had received rich tribute, he sent forth to Babylon."
Babylonian king Nebuchadnezzar doesn't just take Judean king Jeconiah, he also takes with him those left alive from the royal court, most political, cultural, and religious leaders, and around 10,000 other assorted citizens. This is the beginning of the Babylonian Exile. Jeconiah's uncle, Zedekiah, now rules as a puppet of Babylon.