The continued presence of enemy nations within the land was not merely a historical accident, but a deliberate mix of punishment and education for the Israelites. It marked a sharp transition from a life of divine miracles to the harsh realities of the natural world. God allowed these foreign nations to remain to force future generations to confront the heavy price of their wrongdoing. Because the people strayed from God's path, He removed His direct protection, leaving them exposed to the unpredictable forces of nature [מצודת דוד, רלב״ג].
Stripped of this divine cover, the Israelites were suddenly forced to learn the art of war, mastering tactics and battle strategies simply to survive [רש״י, מצודת דוד, רלב״ג, מלבי״ם]. Interestingly, their complete lack of military experience initially led them to seek peace treaties with these enemies rather than engage in combat [ביאור שטיינזלץ].
This new reality stood in stark contrast to the experiences of earlier generations, such as those who lived during the time of Joshua and remained faithful to God [רש״י, מצודת דוד]. Those ancestors never had to study military strategy because God fought their battles for them through open miracles, allowing a single person to chase away a thousand enemies [רש״י, רד״ק, מלבי״ם]. By forcing the current generation to fight ordinary, natural wars, God intended to open their eyes to the incredible miracles their ancestors experienced, hoping this realization would inspire them to return to faith [מצודת דוד, מלבי״ם].
A slightly different perspective suggests that the Israelites' lack of familiarity with these enemies was a matter of status rather than military tactics. In the past, these nations were so weak and insignificant that the Israelites paid them no attention at all. It was only after the Israelites sinned that these once inferior groups gained strength and became a genuine threat, serving as a powerful reminder of how wrongdoing can elevate a forgotten adversary into a dangerous force [חומת אנך בשם כלי יקר].