Judges 3.2 describes, God left those nations simply because He wanted to teach the subsequent generations of Israelites, who had not experienced the earlier battles, how to conduct holy war.
The idea of holy war is not a very positive one today. While it’s quite an old concept the Muslims have made the idea immensely unpalatable. The Muslim version of holy war involves using women, children and the handicapped, in addition to men without other options for gaining heaven, through the means of suicide, for accomplishing their objectives. Their holy war is the destruction of the infidel or the conversion of the infidel through terror and fear. The god they serve is abusive, terrible and unattractive in every way. Yet holy war predates Islam. (more…)