John Gill 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)

And he gathered all Judah and Benjamin, and the strangers with them The proselytes of the gate:

out of Ephraim and Manasseh, and out of Simeon : out of all the places in those tribes that had come off to him, or had been taken by him; for otherwise these belonged to the ten tribes under the government of Jeroboam, and his successors, and the next clause explains it:

for they fell to him out of Israel abundance, when they saw that the Lord his God was with him ; as was clear by the victory he gave him over the Ethiopians; after that time many in the above tribes came over to him; the Targum is, ``when they saw the Word of the Lord his God was his help.''