Albert Barnes Commentary 2 Chronicles 15:9

"And he gathered all Judah and Benjamin, and them that sojourned with them out of Ephraim and Manasseh, and out of Simeon: for they fell to him out of Israel in abundance, when they saw that Jehovah his God was with him." — 2 Chronicles 15:9 (ASV)

Strangers... that is, “Israelites of the tribes of Ephraim and Manasseh.” The separation of the two kingdoms had made their Israelite brethren “strangers,” or “foreigners,” to Judah.