Judges 3:2
Only that the generations of the children of Israel might know, to teach them war, at the least such as before knew nothing thereof; Only that the generations of the children of Israel might know, to teach them war, at the least such as before knew nothing thereof;
The nations were not left in the land by accident or by failure of purpose, but were left deliberately so that the rising generations of Israel might come to know what their fathers had known. Those who had known nothing of warfare were to be taught to fight through their continued presence. The lesson was practical and progressive, giving those without experience the opportunity to learn conflict and conquest firsthand.
The same arrangement reveals that God proves His people at the very points where they have failed before. When they fail again, He brings them back to the same test until the lesson is learned. In this way the remaining nations became both the school of war for the inexperienced and the repeated trial of faithfulness for the whole people.