Albert Barnes Commentary 1 Kings 12:21

"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 12:21 (ASV)

Benjamin's allegiance to Judah at this time comes as a surprise. By blood, Benjamin was far more closely connected with Ephraim than with Judah. All of Benjamin's traditions were antagonistic to Judah, and until now, the weaker tribe had been accustomed to lean constantly on its strong northern neighbor. However, it seems that in the half-century since the revolt of Sheba, son of Bichri (2 Samuel 20:1), the feelings of the Benjamites had completely changed.

This change is best explained by the establishment of the religious and political capital at Jerusalem, located on the border between the two tribes (Joshua 15:8; Joshua 18:16). As a result, the new metropolis stood partly within the territory of each and was, in a sense, common to both. One of Jerusalem's gates was "the high gate of Benjamin" (Jeremiah 20:2), and Benjamites probably formed a considerable part of the city's population. Furthermore, we can reasonably believe the entire tribe was sincerely attached to the temple worship. They could participate in it more freely and constantly than members of more distant tribes, a practice to which the habits of forty years had now accustomed them.

Regarding the number of the Israelites, see the notes on Exodus 12:37 and 2 Samuel 24:9. The number mentioned here is moderate compared with the numbers given both previously and subsequently (2 Chronicles 13:3; 2 Chronicles 17:14–18).