《资治通鉴·唐纪·唐纪五》
我们孤军深入，如果攻城不下，敌人援军从四方赶来，我军就会腹背受敌，进退不成，虽然有船舰又怎么能用？
吾悬军深入，若攻城未拔，援兵四集，吾表里受敌，进退不获，虽有舟楫，将安用之？
We have driven deep into enemy territory. If we fail to capture the city, the enemy reinforcements will come from all sides, and our army will be attacked from both the front and the back, unable to advance or retreat. Even if we had ships, how could we use them?