From Victorious Christian Living by Alan Redpath:

A looming question from Joshua

One of the questions that needs to be answered from the book of Joshua, and I believe can be answered satisfactorily, is the question that seeks to understand and explain why God ordered the destruction of the seven tribes of Canaan.

This post will not attempt a complete answer, but will give a partial explanation.

One of the reasons this wiping out of the inhabitants of Canaan by the Promised Land bound Israelites is delivered in this quote from Alan Redpath's book on Joshua called Victorious Christian Living:

But I would have you observe that they faced not only conflict but victory. God had a purpose for that land. What was it? This-a little babe in a manger at Bethlehem, Christ the Son of God "on a cross at Calvary, one hundred and twenty people in an upper room and the Holy Ghost falling on them. Bethlehem, Calvary, Pentecost: the incamation of the Son of God, the judgment of the sin of humanity heaped on Him, the life of the Son of God incarnate in the the believer; all thin was God's master plan for the salvation of a fallen race. And nothing, I say nothing, on earth or in hell should ever stand in the way of the plan of God. The iniquity of the people was full. Now God begins to act.

Certainly, this is not a complete answer, but it is part of the answer.

God was preparing the redemption of mankind in the life, death, and resurrection of His Son. And this was going to take place in Canaan. And though there are many more facets to the explanation, God's plan of redemption is definitely an important one.

Redpath goes on to say, "The purpose of God for every man and woman is Bethlehem, Calvary, Pentecost, and everything that stands in the way of God's fulfillment of His plan must be conquered."

Too big for God to use

God's Charge to Joshua from the first chapter of Joshua:

No man shall be able to stand before you all the days of your life. Just as I was with Moses, so I will be with you. I will not leave you or forsake you. Be strong and courageous, for you shall cause this people to inherit the land that I swore to their fathers to give them. Only be strong and very courageous, being careful to do according to all the law that Moses my servant commanded you. Do not turn from it to the right hand or to the left, that you may have good success wherever you go. This Book of the Law shall not depart from your mouth, but you shall meditate on it day and night, so that you may be careful to do according to all that is written in it. For then you will make your way prosperous, and then you will have good success. Have I not commanded you "Be strong and courageous. Do not be frightened, and do not be dismayed, for the Lord your God is with you wherever you go.

Alan Redpath's comments on this passage from Victorious Christian Living: Studies in the Book of Joshua:

Most of us, God forgive us, are too big for God to use. We are too full of our own schemes and of our own way of doing things. God has to humble us and break us and empty us. So low, indeed, must God make us that we need every word of encouragement from heaven to enable us to take on the job and dare to go forward in the will of God. The world speaks about the survival of the fittest, but God gives power to the faint and He gives might to those who have no strength. He perfects His strength in weakness; He uses the things that are not to bring to nought the things that are.

Let me not be too big for God to use me; I'd rather be weak and have his encouragement "Be strong and courageous!"