Question: "Why was Abraham promised land that belonged to others (Genesis 12)?"
Answer: In Genesis 12:1-3, the Lord says to Abraham, “Go from your country and your kindred and your father's house to the land that I will show you. And I will make of you a great nation, and I will bless you and make your name great, so that you will be a blessing. I will bless those who bless you, and him who dishonors you I will curse, and in you all the families of the earth shall be blessed.” This blessing included land that, at the time the promise was made, belonged to other people.
There are several reasons why this transfer of ownership was appropriate. First, “the earth is the LORD's, and everything in it, the world, and all who live in it” (Psalm 24:1). As the Creator of the earth, God has the right to do with it as He pleases. He can take land away or give it according to the counsel of His will (Psalm 135:6).
The land pledged to Abraham was part of God’s provision for the Jewish people. After the Exodus from Egypt, the Jews were given the Promised Land, confirming God’s power to predict the future and fulfill His promises.
Second, giving the land to Abraham’s descendants was, in part, a judgment on the sinful Canaanites. In Genesis 15:16 the Lord gives a timeframe for the transfer of the land, as well as a reason for it: “In the fourth generation your descendants will come back here, for the sin of the Amorites has not yet reached its full measure.” This statement clearly shows that God had a reason for wresting the land from the Canaanites—namely, their sin. At the edge of the Promised Land, Moses told the children of Abraham, “It is on account of the wickedness of these nations that the LORD is going to drive them out before you” (Deuteronomy 9:4). Abraham did not inherit the land immediately because it was not time yet for judgment to fall. God eventually took the land from the idolaters and turned it over to His children.
Third, the prosperity promised to Abraham required much land. Prosperity in Abraham’s time involved acquiring land and having much livestock. God’s promise to make Abraham prosperous would virtually require giving him large amounts of land.
Fourth, the geographical portion of the Abrahamic Covenant served as the historical basis for Israel’s eventual settlement of the land. Though there were many nations living in Canaan when Israel crossed the Jordan River, God’s promise to Abraham was Israel’s claim to the land. In Genesis 15:18-21, God further defined the borders of the land promised to Abraham: “To your offspring I give this land, from the river of Egypt to the great river, the river Euphrates, the land of the Kenites, the Kenizzites, the Kadmonites, the Hittites, the Perizzites, the Rephaim, the Amorites, the Canaanites, the Girgashites and the Jebusites.”
Yes, God promised Abraham land that belonged to others. The reasons for this transfer of land include the need to punish the Canaanites’ sin and the need for God’s chosen people to have a land of their own, eventually to become the birthplace of the Messiah.