God performed two tremendous miracles to bring the Israelites into the Promised Land: the heaping up of the Jordan River, as the people crossed, and the destruction of Jericho. But from that time on, they had to fight for everything they got. This is true in the Christian life, as well. God will do certain miracles to bring you in. After that, you get only what you fight for, and if you do not fight for it, you do not obtain it!
Historically the Israelites did not take over the whole land at that time. They coexisted with the alien peoples—which was a disaster for them. This, too, is a picture of the Church, trying to move in and coexist with enemy forces that should not be there.