Even though Rehoboam had fled to Jerusalem, he had no intention of letting the rebellion go on. He decided to take military action. He, therefore, mustered an army made up of soldiers from the tribes of Judah and Benjamin.
Parenthetically, I might say that the inclusion of Benjamin may give us the answer to the one strip of cloth for which there is no accounting in the words of the prophet Ahijah to Jeroboam in 1 Kings 12:1–33. There is, however, another possibility later in the book at which point the writer tells us that priests and Levites forsook their cities and pasture lands in the north, because Jeroboam had appointed other priests not of the tribe of Levi to be in charge of the kingdom’s worship. That presents the possibility that the unnamed piece of Ahijah’s garment symbolized the tribe of Levi.
The size of the army gathered together was 180,000 men. This number probably does not represent the maximum number of soldiers that could be assembled. Instead, it may be the maximum that could be prepared on short notice. The hope for Rehoboam was to bring a sizable force against the northern tribes before Jeroboam had a chance to consolidate his rule.
21 When Rehoboam came to Jerusalem, he assembled all the house of Judah and the tribe of Benjamin, 180,000 chosen warriors, to fight against the house of Israel, to restore the kingdom to Rehoboam the son of Solomon.