“These are the kings of the land.” The Book of Joshua now switches its attention from the territory east of the Jordan River to the territory west of the Jordan River, the land promised to Abraham west of the Jordan River. Joshua 12:1-6 described the territory that Israel inherited east of the Jordan River, and now Joshua 12:7-24 describes the territory that Israel inherited west of the Jordan River.
“Joshua and the children of Israel struck.” Like Moses was on the east side of the Jordan, Joshua is now Yahweh’s representative, and ultimately it is Yahweh who gives the land to Israel (cp. Num. 14:8, Joshua 1:2, 11, 8:1, etc.). Some of these places were conquered and lived in. Others were just conquered but Joshua’s army moved on and the Israelites were not able to occupy them at the time, and sometimes in those cases the Canaanites moved back into the cities and they had to be conquered again later.
“westward.” The Hebrew can also be understood as “toward the (Mediterranean) Sea.”
“from Baal-gad in the valley of Lebanon even to Mount Halak that goes up to Seir.” Baal-gad is in the north part of Israel, while Mount Halak is in the far south of Israel (cp. Josh. 11:17).
“Joshua gave it.” The “it” is the land, as is clear from the context.