Charles Ellicott Commentary 1 Kings 12:20-21

"And it came to pass, when all Israel heard that Jeroboam was returned, that they sent and called him unto the congregation, and made him king over all Israel: there was none that followed the house of David, but the tribe of Judah only. And when Rehoboam was come to Jerusalem, he assembled all the house of Judah, and the tribe of Benjamin, a hundred and fourscore thousand chosen men, that were warriors, to fight against the house of Israel, to bring the kingdom again to Rehoboam the son of Solomon." — 1 Kings 12:20-21 (ASV)

In these two verses, we again have the same curious juxtaposition of the tribe of Judah only and the house of Judah, with the tribe of Benjamin.

The army gathered would, no doubt, be drawn from Solomon’s established and disciplined forces, as well as from the general levy of Judah and Benjamin. It perhaps also included contingents from the tributary races (as in 2 Samuel 17:27), who would bear a strong personal allegiance to the house of Solomon, and be prepared to stamp out the rebellion before it could thoroughly organize itself for disciplined resistance.