Biblical Sources concerning the need to expel alien antagonistic elements
Numbers (NKJV) 33:
53 You are to take possession of the land and settle in it, for I have given you the land to possess. ..
55 But if you do not drive out the inhabitants of the land from before you, then it shall be that those whom you let remain shall be irritants in your eyes and thorns in your sides, and they shall harass you in the land where you dwell. 56 Moreover it shall be that I will do to you as I thought to do to them.
Deuteronomy (NKJV) 7:
22 And the LORD your God will drive out those nations before you little by little; you will be unable to destroy them at once, lest the beasts of the field become too numerous for you. 23 But the LORD your God will deliver them over to you, and will inflict defeat upon them until they are destroyed.
Exodus (NKJV) 23:
29 I will not drive them out from before you in one year, lest the land become desolate and the beasts of the field become too numerous for you.
30 Little by little I will drive them out from before you, until you have increased, and you inherit the land.
Joshua (NKJV) 23:
5 And the LORD your God will expel them from before you and drive them out of your sight. So you shall possess their land, as the LORD your God promised you.
Three Reasons why the Israelites Could Not Drive Out the Canaanites At Once.
See the Brit-Am Commentary (BAC) to Genesis 15:16:
http://britam.org/Genesis/Gen12to17LechLacha.html
1. The iniquity of the Canaanites (Amorites) was not yet complete (Genesis 15:16).
2. To Teach the Israelites WAR (Judges 3:2).
3. In order that the land not become a waste the Amorites were to be left in their cities until the Israelites become numerous enough to occupy all of the area (Deuteronomy 7:22).