Rehoboam lived in Jerusalem, and he built cities for defense in Judah.
How the ten tribes deserted the house of David we read in the foregoing chapter. They had formerly sat loose to that family (Sa2 20:1, Sa2 20:2), and now they quite threw it off, not considering how much it would weaken the common interest and take Israel down from that pitch of glory at which it had arrived in the last reign.
Commenting on 2 Chronicles 11:1-12
He built even Bethlehem,.... The place of our Lord's birth, about six miles from Jerusalem: and Etam; another city in the tribe of Judah; see Ch1 4:32, there was a rock of this name, Jdg 15:8 and a fountain from whence waters were conveyed into the pool at Jerusalem, spoken of in Jewish writings (b): and Tekoah; of which place Amos the prophet was, and...
built cities for defence in Judah--This is evidently used as the name of the southern kingdom. Rehoboam, having now a bitter enemy in Israel, deemed it prudent to lose no time in fortifying several cities that lay along the frontier of his kingdom. Jeroboam, on his side, took a similar precaution (Kg1 12:25).