This phrase is used here to mark an important event in the story. If your language has a way for doing this, you could consider using it here.
“All Israel” here is a generalization that means the capable men of Israel who represent the rest of the people by a synecdoche. AT: “when all the leaders of Israel heard” (See: rc://en/ta/man/translate/figs-synecdoche and rc://en/ta/man/translate/figs-hyperbole)
“Israel” implicitly means the northern ten tribes that rebelled against Rehoboam. AT: “king over all of the 10 tribes of Israel” (See: rc://en/ta/man/translate/figs-explicit)