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; Ezekiel 21:25.
Related Commentaries of 2 Chronicles 36:13
2 Chronicles 36:13
13 And he also rebelled against king Nebuchadnezzar, who had made him swear by God: but he stiffened his neck, and hardened his heart from turning unto the LORD God of Israel.