Jamieson, Fausset and Brown on Ezekiel 21:20

20. Rabbath of the Ammonites--distinct from Rabbah in Judah (2Sa 12:26). Rabbath is put first, as it was from her that Jerusalem, that doomed city, had borrowed many of her idols.

to Judah in Jerusalem--instead of simply putting "Jerusalem," to imply the sword was to come not merely to Judah, but to its people within Jerusalem, defended though it was; its defenses on which the Jews relied so much would not keep the foe out.