With the tribe of Benjamin, i.e. that part of it which was next to Judah, and joined with them. See Poole on "1 Kings 11:13". Against the house of Israel, i.e. the families or tribes (for these words are promiscuously used one for the other) of Israel.
Related Commentaries of 1 Kings 12:21
1 Kings 12:21
21 And when Rehoboam was come to Jerusalem, he assembled all the house of Judah, with the tribe of Benjamin, an hundred and fourscore thousand chosen men, which were warriors, to fight against the house of Israel, to bring the kingdom again to Rehoboam the son of Solomon.