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After Solomon died, the northern tribes turned from his son Rehoboam at Shechem and made Jeroboam king. Jeroboam had come back from Egypt, and Rehoboam fled to Jerusalem. From then on the north was called Israel and the south Judah. The Bible says Benjamin stayed with Judah (1 Kings 12:21). The map's line runs close to Jerusalem, so it puts much of Benjamin's land in Israel.
Read it: 1 Kings 12:1-24; 2 Chronicles 10:1-19