"When you approach a city to fight against it, you shall offer it terms of peace." (Deuteronomy 20:10)
Chapter 20 of the book of Deuteronomy begins with "When you go out to battle against your enemies and see horses and chariots and people more numerous than you, do not be afraid of them; for the Lord your God, who brought you up from the land of Egypt, is with you." The entire chapter focuses on how the Israelites should behave during times of war.
This commandment literally says we should offer terms of peace to a city when we approach it. Note that this is before any attack is made on the city.
Messiah implicitly affirmed this commandment when He spoke about the Law:
"Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven. (Matthew 5:17-19)
This may also have been in mind when Messiah declared:
Blessed are the peacemakers, for they shall be called sons of God. (Matthew 5:9)
This commandment reflects the character of Messiah and His grace. He extends peace to His enemies before the battle.
G-d warned Adam in the Garden of Eden of the consequences of disobedience ("for in the day that you eat from it you will surely die"- Genesis 2:17). He warns Israel of the consequences of disobedience ("and you will perish quickly from the good land which the Lord is giving you."- Deuteronomy 11:17). Over and over again, we find G-d warning people and giving them the opportunity to avoid sin. He offers them terms of peace: obey Me and live!
When Messiah came into the world, the entire world was His enemy: "He was in the world... and the world did not know Him." (John 1:10). He came to His own people and they did not receive Him (John 1:11). While we were still sinners, Messiah died for us (Romans 5:8).
Messiah was the ultimate peacemaker and redeemed humanity to G-d. "We have peace with G-d through our Lord Yeshua the Messiah." (Romans 5:1)
Messiah did not fulfill this commandment because He did not approach a city to fight against it.
This commandment is observed in a literal fashion: offer terms of peace when approaching a city to besiege it.
Revelation 14:8 ("Fallen, fallen is Babylon the great...") and other passages provide warning to those cities and nations who oppose G-d. He has foretold of their destruction if they do not repent. Much like Nineveh in the book of Jonah, they have been warned.
We are able to fulfill this commandment today and when we approach a city to fight against it, we should offer it terms of peace.