דברי הימים ב, פרק ט״ו, פסוק ט׳

The Kingdom of Judah, which previously consisted exclusively of the tribes of Judah, Benjamin, and the Levites, experiences a major shift in its population as people from other tribes begin to arrive [ביאור שטיינזלץ]. This mass gathering takes place in the aftermath of a decisive military victory. During the fifteenth year of his reign, King Asa engages in a war against Baasha, the King of Israel. Asa emerges victorious, claiming substantial spoils from the conflict. Following this triumph and the prophetic message delivered by Azariah the son of Oded, a widespread migration of Israelites into Asa's kingdom begins [רלב״ג].

Members of the tribes of Ephraim, Manasseh, and Simeon abandon their former homes to align with Asa. Rather than a forced surrender or physical collapse, their movement represents a deliberate shift in loyalty. They willingly gravitate toward Asa, choosing to accept his rule and place themselves completely under his government [מצודת ציון].

The primary motivation for this relocation is the clear recognition of Asa's success in his war against Baasha. Observing that God is undeniably with him, the crowds act on the common human tendency to align with the stronger, victorious side, ultimately choosing to unite their destiny with the Kingdom of Judah [ביאור שטיינזלץ].