All these kings and their land did Joshua take at one time, because Yahweh, the God of Israel, fought for Israel.
God’s people do not have to engage every enemy that exists immediately, any more than the Israelites had to kill every individual Canaanite or attack every city immediately. At this stage in their national life God’s will was that they attack only certain selected targets. Sometimes we can experience discouragement when we look at the host of wicked people that surrounds us, or the many sins that plague us. We may think, "What can one individual do to stem such a tide of wickedness?" We may even think it is useless to do anything in view of the huge task we face (Matthew 28:19-20). In those situations we need to do what God puts before us to do day by day rather than taking on more responsibility than God wants us to assume immediately (cf. Matthew 6:25-34).