Albert Barnes Commentary 2 Chronicles 36:13

"And he also rebelled against king Nebuchadnezzar, who had made him swear by God: but he stiffened his neck, and hardened his heart against turning unto Jehovah, the God of Israel." — 2 Chronicles 36:13 (ASV)

The oath of allegiance was taken when he was first installed in his kingdom. On Zedekiah’s sin in breaking his oath, see Ezekiel 17:18-20 and Ezekiel 21:25.