The land was originally inhabited by Canaanites. In Exodus 3:8 God refers to the land as "Land of the Canaanites". Inhabited by Hitties, Amorities, Perizzites, Hivites, Jubsites. Maybe others also.

However, God gave the land to Abraham and all his seed thereafter for a permanent possission.

Why did God give away the land that belong to another? Because of sin.
The residents of Canna worshiped idols and such. To God this was sin. The Hebrew's now called Israelies obeyed God. So as directed by God, Joshua conquered the land and overthrew God's enemies.

Israel has lost some ownership over the years due to their sin and dis-obidence to God, however, their rights are as "everlasting possession". The Israelites are still God's chosen people.

God created the earth. He owns the land and as owner he can give it to who he wants.

I base all of the above on the Holy Bible. Oh, yes. I know many of you do not accept the Holy Bible. However, you can not show any evidence or true reference to dispute the above.