God warns/counsels the people, “No civil war.” Rehoboam is all over the place. He’s bad for awhile, good for a few years, then back to being evil. Ultimately, he ends up being deemed a bad king.
Egypt plunders Jerusalem. It’s the people’s first taste of slavery to the Egyptians since the Exodus. It doesn’t bode well with them. They cry out for help.
Abijah takes over Judah after Rehoboam dies. He makes civil war, calling out Jeroboam and the Israelites, saying God is on Judah’s side NOT Israel’s. Apparently, Abijah and Judah have a good point, for they slaughter over 1/2 of Israel’s army in battle.
CONCLUSION: Both Judah and Israel rebel. The difference is that Judah repents; Israel does not.