until Yahweh has given your brothers rest, as he has given you, and they have also possessed the land which Yahweh your God gives them. Then you shall return to the land of your possession, and possess it, which Moses the servant of Yahweh gave you beyond the Jordan toward the sunrise.’”
Joshua recalled the condition Moses had set when he granted them the eastern land: they must fulfill their oath to fight. The word "all" should not be pressed, only about 40,000 of the 110,000 fighting men crossed Jordan; the rest remained to defend their families, flocks, and territory they had taken.
AI summary
Commenting on Joshua 1:12-18
Until the LORD have given your brethren rest, as [he hath given] you, and they also have possessed the land which the LORD your God giveth them: then ye shall return unto the land of your possession, and enjoy it, which Moses the LORD'S servant gave you on this side Jordan toward the sunrising. (i) By your request, but yet by God's secret appointment, .
Until the Lord hath given your brethren rest, as he hath given you,.... Rest from their travels, as they had: and they also have possessed the land which the Lord your God giveth them: are settled in the land of Canaan, as they were on that side Jordan: then ye shall return unto the land of your possession, and enjoy it; the countries of Sihon...