Refer to 32:9:2:1.

Rashi citing Targum Yonasan: They are serene countries.

Malbim: This is like "Hoy ha'Sha'ananim b'Tziyon veha'Botechim b'Har Shomron" (Amos 6:1). Sha'anan is one who dwells serene, even if he does not trust in his Gevurah, because no one fights him. Bnei Tziyon did not trust in their Gevurah; they were serene because Sancheriv did not fight with them. Bnei Efrayim were not Sha'ananim, for Sancheriv fought them, but they trusted in their Gevurah. Bnei Tziyon and Yehudah are called Nashim, for Malchus Yehudah began in the days of David; Malchus Efrayim separated later, in the days of Rechav'am. This is like a woman who gave birth to a daughter who rules.

Brachos 17a: Hashem made a greater promise to women than to men (they are calm and assured).

Rashi citing Targum Yonasan: Great cities that dwell secure. Radak - this refers to Arei Yehudah. The Navi tells them, even though now you are secure, you will be agitated.

Malbim: Refer to 32:9:1:3.