A cruel and unusual condition of surrender is presented to a desperate city, reflecting a desire that goes far beyond simple military conquest. The enemy seeks a profound national and spiritual humiliation.
The absolute and only term for a peace treaty [מצודת דוד, ביאור שטיינזלץ] is the horrific demand to gouge out and remove [מצודת ציון] the right eye of every resident [רש״י, רד״ק]. Typically, a treaty between nations serves to prevent war through a mutual exchange of benefits. However, the townspeople are completely stripped of military power and have nothing of value to offer in return for their safety. Because they are utterly defenseless, the only benefit the enemy extracts from this agreement is the pleasure of their abuse, turning them into objects of mockery [מלבי״ם]. The ultimate goal is to cast a severe disgrace upon the entire nation of Israel [רש״י]. If the Israelites fail to rescue their brothers from such a terrible fate, it will publicly expose the weakness and helplessness of the whole nation [מלבי״ם, מצודת דוד].
Alongside this literal threat, there is a deeper spiritual dimension to the demand for the right eye. Rather than seeking to destroy a physical body part, the enemy demands that the Israelites hand over the Torah scroll, which was given from God's right hand. His intention is to uproot and burn the specific law that permanently forbids Ammonites and Moabites from entering the assembly of God [רש״י, רד״ק, אלשיך].
This symbolic interpretation is rooted in the deliberate emphasis on the enemy's identity as an Ammonite. Because his origin is highlighted again despite being established earlier, it reveals that his cruel demand stems directly from his anger over the Torah disqualifying his people. Furthermore, the specific targeting of the right eye perfectly parallels the structure of the Torah's prohibition, where the Ammonite is listed first, on the right side, just before the Moabite [אלשיך].