When Rehoboam had established the kingdom. — Rather, when Rehoboam’s kingdom had been established. The construction is impersonal: when one had established Rehoboam’s kingdom. The narrative is resumed from 2 Chronicles 11:17.
And had strengthened himself. — And when he had become strong (hezqâh, an infinitive, used again at 2 Chronicles 26:16; Daniel 11:2, and nowhere else).
He forsook the law of the Lord — i.e., lapsed into idolatry. (See 1 Kings 14:22-24, where the offence is more precisely described.)
All Israel. — The southern kingdom being regarded as the true Israel. (Comp. 2 Chronicles 12:6.)