John Gill Commentary 2 Chronicles 14:6

"And he built fortified cities in Judah; for the land was quiet, and he had no war in those years, because Jehovah had given him rest." — 2 Chronicles 14:6 (ASV)

And he built fenced cities in Judah
For his defence against the kingdom of Israel and other nations, as Rehoboam had done before him; and which might have been demolished by Shishak king of Egypt, when he took them, (2 Chronicles 11:5–12) (12:4)

for the land had rest ;
according to the Targum, the land of Israel rested, and gave no disturbance to the kingdom of Judah, not having recovered the blow given them by Abijah; but it is rather to be understood of the land of Judah, which, as it did not attempt the reduction of the ten tribes, so it was neither attacked by them, nor any other enemy: and he had no war in those years; in the ten years mentioned, (2 Chronicles 14:1) , neither with Israel nor any other nation: because the Lord had given him rest; that he might be at leisure to do the above things; all rest is from the Lord, civil, spiritual, and eternal.