Therefore: This word introduces two divine decrees. The first decree condemned Babylon to complete destruction, like the punishment the Lord gave Assyria (Nahum 3). Assyria fell in 612 BC; Babylon fell in 539 BC.
I will return Israel: The second divine decree promises to restore Israel, specifically the people of the northern tribes, whose land was defined by the mentioned landmarks.
Carmel is a mountain range near the Mediterranean Sea in the northwestern corner of Israel.
Bashan was a region in the highlands east of the Sea of Galilee.
The tribe of Ephraim lived in the hill country in central Israel west of the Jordan River.
Gilead was a highland area on the east side of the Jordan River, opposite Ephraim.