“And when Rehoboam was come to Jerusalem, he gathered of the house of Judah and Benjamin an hundred and fourscore thousand chosen men, which were warriors , to fight against Israel, that he might bring the kingdom again to Rehoboam.”
That is, the half tribe of Benjamin: for the other half was gone after Jeroboam. Cross references: : 1 Kings 12:20 , 21 Meaning the ten tribes which rebelled.