What does leave nothing to invite the enemy to return destroy whatever cannot be consumed let the valley be left so that crows flying over it will have to carry their rations along with them mean?

This phrase suggests a strategy of total annihilation or denial of resources to an enemy, ensuring they cannot reclaim or benefit from the land. By destroying everything that cannot be consumed, it emphasizes a scorched-earth approach, leaving nothing of value for the adversary. The imagery of crows needing to carry their rations implies that even nature is stripped of sustenance, reinforcing the idea of complete desolation. Ultimately, it conveys a relentless commitment to preventing the enemy's return and survival.