When Rehoboam arrived in Jerusalem, he mustered the house of Judah and Benjamin—a hundred and eighty thousand fighting men—to make war against Israel and to regain the kingdom for Rehoboam.
But this word of the LORD came to Shemaiah the man of God: "Say to Rehoboam son of Solomon king of Judah and to all the Israelites in Judah and Benjamin, 'This is what the LORD says: Do not go up to fight against your brothers. Go home, every one of you, for this is my doing.' " So they obeyed the words of the LORD and turned back from marching against Jeroboam. For some crazy reason that I'm probably never gonna figure out, GOD CAUSED THAT!
The priests and Levites from all their districts throughout Israel sided with him. The Levites even abandoned their pasturelands and property, and came to Judah and Jerusalem because Jeroboam and his sons had rejected them as priests of the LORD. And he appointed his own priests for the high places and for the goat and calf idols he had made. Those from every tribe of Israel who set their hearts on seeking the LORD, the God of Israel, followed the Levites to Jerusalem to offer sacrifices to the LORD, the God of their fathers. They strengthened the kingdom of Judah and supported Rehoboam son of Solomon three years, walking in the ways of David and Solomon during this time. I find it strange that all the good guys went over to his side during this whole thing. I guess It's just that while Jeroboam was evil, Rehoboam was immature, though he still followed God.