During Yaakov Avinu’s final brocho to his children, he tells Yehuda, “Lo yosur shevet m’yehudah,” meaning, “The staff will not leave from Yehudah.” Chaza”l teach us that this is not merely a prophecy, but a halacha that it is forbidden to appoint a permanent king from any tribe besides Yehudah. Interesting to note, the Ramba”n states that this was the reason for the tragic end of the Chashmonai family. Eventually, they took the kingship of Klal Yisroel for themselves. Since they were Kohanim, and not from the tribe of Yehuda, they had no right to the kingship and were punished severely for this transgression.
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Rabbi Aharon Dovid Singer
Rosh Kollel, NMB Kollel