The enemies of Judah sought to undermine the reconstruction of Jerusalem's walls by publicly mocking the builders and their efforts. Standing closely alongside Sanballat, perhaps acting as one of his deputies [ביאור שטיינזלץ], Tobiah the Ammonite joined in the chorus of ridicule [מצודת דוד, רש״י]. He declared that even if the Jewish people managed to complete their construction, the resulting structure would be incredibly flimsy. It would be so unstable that it could never function as a true fortress [מלבי״ם, ביאור שטיינזלץ].
To highlight his contempt, Tobiah claimed that the mere presence of a fox climbing on the wall would be enough to cause the stones to crumble. This illustrated his belief that the wall completely lacked structural integrity, making it easy for anyone to knock it down at will [רש״י].
The specific choice of a fox in this insult carries several layers of meaning. On a practical level, foxes are known for their habit of digging and creating holes within ruins [מלבי״ם]. Furthermore, the fox is viewed in this context as a weak and slow creature [רלב״ג]. Tobiah used this imagery to argue that if such a feeble animal could break through their defenses, then certainly mighty beasts like lions, or powerful empires like the Chaldeans and Persians, would be able to destroy the wall effortlessly [רס״ג]. Beyond the physical insult, the mention of a fox was likely a calculated and cruel emotional strike. By invoking the image of foxes roaming the area, Tobiah was reminding the builders of the tragic lament over the ruined Mount Zion, which was famously described as a desolate place where foxes prowl [רס״ג].