David and Solomon fortified Jerusalem, Judah, and all of Israel, but when Solomon’s son Rehoboam took the throne, he made a huge mistake, which divided the nation. Ten tribes broke off, which meant that the king lost many defensive positions and soldiers.

So Rehoboam dwelt in Jerusalem, and built cities for defense in Judah…And he fortified the strongholds, and put captains in them, and stores of food, oil, and wine. Also in every city he put shields and spears, and made them very strong, having Judah and Benjamin on his side (2 Chronicles 11:5–12).

Jerusalem, Judah, and Benjamin had defenses, but Rehoboam had to make them stronger, because he had a new enemy, which was Jeroboam and Israel in the North.

Defenses are good, but sometimes we have to make them stronger.