This was given to Israel as they were in the wilderness, preparing to enter and conquer the land of Canaan. For us today, it has another meaning. The land being conquered is not the country around us, but the country within. When we say to Jesus, "Lord, I want you to come in and rule my life," then all other principalities and powers, all other rulers and authorities, must be removed. This is the land of our heart, and you cannot serve God and anything else. Jesus is either Lord of all, or he is not Lord at all.
Jesus said we cannot serve God and materialism. He also told us that whoever seeks their own life will lose it, but whoever gies up their life for the gospel will find it. "America First" may (or may not) be a good political slogan, but for the follower of Jesus, it has to be "Jesus First....and Only."
So the question I ask myself, as I think about these words from Numbers, am I allowing, even helping, Christ to remove all those (hopefully) former inhabitants of my heart?