When Rehoboam arrived in Jerusalem, he mobilized the house of Judah and Benjamin—180,000 chosen warriors—to fight against Israel and restore the kingdom to Rehoboam.
How the ten tribes deserted the house of David we read in the foregoing chapter. They had formerly sat loose to that family (Sa2 20:1, Sa2 20:2), and now they quite threw it off, not considering how much it would weaken the common interest and take Israel down from that pitch of glory at which it had arrived in the last reign.
Commenting on 2 Chronicles 11:1-12
And when Rehoboam was come to Jerusalem,.... After he had been at Shechem, and had given his answer to the request of the men of Israel, upon which they revolted from him: this and the three following verses are the same with Kg1 12:21. See Gill on Kg1 12:21. . 2 Chronicles 11:5 ch2 11:5 ch2 11:5 ch2 11:5And Rehoboam dwelt in Jerusalem,....
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. (a) That is, the half tribe of Benjamin for the other half went after Jeroboam. (b) Meaning the ten tribes who rebelled.