“you will cause.” God works through people. God supplied much, but the work Joshua did was still important for success. God and His human agents work together to accomplish His purposes. As Paul wrote, “we are God’s fellow workers” (1 Cor. 3:9). There are times when God says He would cause the people to inherit the land (cp. Josh. 1:11), and there are times such as here when God says His human agents do that (Deut. 31:7; Josh. 14:1).
“to inherit.” The Hebrew term for “inherit” is from a root word that is the same root word for “valley.” To get an inheritance was to get land.
“I swore.” God swore to give the land. He promised. Thus the land is the “Promised Land.” One of the many themes in Joshua is that God keeps His promises, and thus He is giving the “Promised Land” to Israel because He swore to the “fathers” (primarily referring to Abraham, Isaac, and Jacob) that He would give it to their descendants.