Deuteronomy 2:19
And when thou comest nigh over against the children of Ammon, distress them not, nor meddle with them: for I will not give thee of the land of the children of Ammon any possession; because I have given it unto the children of Lot for a possession.
Israel when coming close to the Ammonites was to leave them undisturbed and unprovoked, taking none of their land for itself, because that land had been given to the children of Lot as their own possession. The restriction marked a divinely set boundary on the conquest, so that territory granted to Lot's line was exempt from seizure while nations wholly abandoned to evil were to be destroyed instead of being spared to spread corruption to others.
The command stands among the proofs that God's covenant with literal Israel rested on obedience rather than on descent alone. On the condition of hearkening to the judgments and keeping and doing them, God would keep toward Israel the covenant and mercy sworn to their fathers and exalt them high above all nations as His favored people for their fathers' sakes. If instead they refused to hearken so as to do all His commandments and statutes, mere Abrahamic lineage could not make them His people, and all the curses would come upon them and overtake them.