The list of hostile peoples in Judg 3:5 is the same as the traditional list of enemies intended for destruction (Ex 33:2).  This means that the effect of the Israelite invasion fell far short of expectations.  Despite thousands of casualties and scores of towns being taken, not a single enemy tribe appears to have been wiped out.  This was due not to a change in God's heart, but to the lack of will of the Israelites.  They were tired of war, and wanted to become farmers and herdsmen.  Caleb was always ready for a battle (Judg 1:12-3), but by and large the Israelites preferred to settle on their allotments and enjoy the fruits of what they had already won.

         

Who can blame them? They had been through many years of warfare and wandering, raising their families in tents, constantly moving.  Even when they were assigned a territory for their tribe or clan, the men were called away from home to do battle in Israel's armies to expand the area of settlement.  It was so much easier to stay home and build up a home, a family, a farm, a town.  The local people weren't all that bad, either, no need to drive them away, let alone kill them!  For one thing, they were useful in doing manual labor, and some of their women were really good-looking.  They had their own customs, of course, their own religious beliefs and practices, but they didn't try to force them on the Israelites.  Really, co-existence was much move civilized than conquest!

         

This is why the start of chapter 3 strikes such a contrary note:

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          These are the nations the Lord left to test all those Israelites who had not experienced any of the wars in Canaan (he did this only to teach warfare to the descendants of the Israelites who had not had previous battle experience): the five rulers of the Philistines, all the Canaanites, the Sidonians, and the Hivites... living in the Lebanon mountains from Mount Baal Hermon to Lebo Hamath.  They were left to test the Israelites to see whether they would obey the Lord's commands (Judg 3:1-4).    

          

The purpose of leaving the other nations in Canaan was to "teach warfare" and to test the inexperienced (younger) Israelites!   This is one of those moments of incomprehensibility for the modern reader.  After all, what is the social justification for religion if not to promote peace?  Many in Western society have become obsessed with the idea of peace at all costs.   For them, the purpose of leaving the foreign nations in Canaan would have been to teach the Israelites inclusivity, tolerance and mutual respect. This kind of reader cannot help but miss the entire point of God's purpose in history, which is fundamentally exclusive:  "All have sinned," says Romans 3.  The sentence of death is pronounced over all mankind.  Inclusivity is by grace, by invitation, by mercy, but not by right.

          

God's focus is not on peace, but on purpose, His purpose of nation-building.   Prior to the Exodus, He selected the few and discarded the many, so as to preserve the Promise in its purity -- the Promise of being God's own people, distinct from all other families of the earth.  After the Exodus, He still taught the people to discriminate -- between the holy and the common, the clean and the unclean, obedience and rebellion.  He wanted to train them to be strong in righteousness, zealous for His Name, intolerant of idolatry.  But Israel yearned only to be as fat and lazy as their sheep.

         

Therefore, if they did not have the spirit to rid the land of their enemies, God would make use of those enemies to provoke His people to change.  If they would not fight offensively, then they would have to fight defensively, when their tolerated enemies turned against them and attacked them. It was the long way round the mountain, but it would work eventually -- not in achieving lasting peace, but in developing a people who valued their identity as His own people.  Because the reality was -- they were not like everyone else, they were not just another tribe amongst the Canaanites.  They were God's representatives on earth.