Deuteronomy 33:21

21 He chose the best land for himself; the leader’s portion was kept for him. When the heads of the people assembled, he carried out the LORD’s righteous will, and his judgments concerning Israel.”

Read Deuteronomy 33:21 Using Other Translations

And he provided the first part for himself, because there, in a portion of the lawgiver, was he seated; and he came with the heads of the people, he executed the justice of the LORD, and his judgments with Israel.
He chose the best of the land for himself, for there a commander's portion was reserved; and he came with the heads of the people, with Israel he executed the justice of the LORD, and his judgments for Israel."
The people of Gad took the best land for themselves; a leader’s share was assigned to them. When the leaders of the people were assembled, they carried out the LORD ’s justice and obeyed his regulations for Israel.”

What does Deuteronomy 33:21 mean?

John Gill's Exposition of the Bible
Deuteronomy 33:21

And he provided a first part for himself
That is, the portion of the land of Sihon and Og, as Jarchi rightly interprets it; which was the beginning or firstfruits of the subduing of the land that was promised; this he looked at, chose, and desired it as his inheritance, ( Numbers 32:1 Numbers 32:2 ) ;

because there, [in] a portion of the lawgiver, [was he] seated;
or in the portion and part of the inheritance of Israel was he placed by Moses the lawgiver, according to the will of God; or because there were in it palaces and towers of great personages, lawgivers, kings, and princes, well covered and strongly fortified; or here he was "hid", or "protected", i.e. their families, wives, and children, while they assisted their brethren in subduing Canaan:

and he came with the heads of the people;
either "to" them, to Moses, Eleazar, and the seventy elders, and the heads of the tribes of Israel, to ask leave to have his part and portion on the other side Jordan, or "with" them, as we supply it; came with them over Jordan armed, to assist in the conquest and subduing of the land of Canaan:

he executed the justice of the Lord, and his judgments with Israel;
that is, either he justly and truly fulfilled all his promises and engagements, on condition of which he was placed on the other side; or he, together with the rest of the Israelites, executed the righteous judgment of God upon the Canaanites, in the expulsion and destruction of them.

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