John Gill Commentary 2 Chronicles 17:2

"And he placed forces in all the fortified cities of Judah, and set garrisons in the land of Judah, and in the cities of Ephraim, which Asa his father had taken." — 2 Chronicles 17:2 (ASV)

And he placed forces in all the fenced cities of Judah
A considerable number of soldiers, to defend them should they be attacked:

and set garrisons in the land of Judah :
on the frontiers and borders of it, to protect it:

and in the cities of Ephraim ;
soldiers to keep garrison there:

which Asa his father had taken ;
see (2 Chronicles 15:8) .