Albert Barnes Commentary Joshua 17:1

"And [this] was the lot for the tribe of Manasseh; for he was the first-born of Joseph. As for Machir the first-born of Manasseh, the father of Gilead, because he was a man of war, therefore he had Gilead and Bashan." — Joshua 17:1 (ASV)

Manasseh, as the “first-born,” was to receive not only the territory on the east of Jordan won by the valor of the Machirites, but also a portion with the other tribes on the west of Jordan, the holy land of promise strictly so called. Thus, though Ephraim took precedence of Manasseh, according to the prediction of Joseph (Genesis 48:20), yet Manasseh received “the double portion” which was the special privilege of the first-born (Deuteronomy 21:17).