The Israelites are commanded not to fight against Edomites, Moabites, or Ammonites.
After the Israelites skirted Mount Seir, the traditional home of the Edomites, they were told to set out. Before their departure, though, God instructed the Israelites to be very careful not to provoke the Edomites. God would not give the Israelites any of their land, not so much as a footprint. God had displaced the Horites so that Edom could settle in Seir, and the Israelites were not to displace the people whom God had settled.
Similarly, the Israelites were told not to provoke the Moabites or Ammonites, because God had given them their land as a possession.
The rationale for Israelite caution to not provoke the other nations was twofold, one theological, and one anthropological. Already mentioned, God had settled the peoples where God intended, and displaced people whom God intended to replace. Just as God would not allow God promised that Abraham would become the father of a great nation, receive a land, and bring blessing to all nations. More to displace Amorites before they were fully guilty and liable for displacement (Genesis 15:16), God did not take displacement or conquest lightly for any of the Israelites’ new neighbors.
Secondly, Deuteronomy reminds the Israelites that other nations are their family. The Edomites are brothers (2:4). The Moabites and Ammonites are sons of Lot (2:9, 19), Abraham’s nephew, to whom Abraham granted their land to reduce conflict (Genesis 13:5-13).
Deuteronomy does not envision a conquest of opportunity, with the Israelites taking over whatever lands they can and attacking neighbors indiscriminately. Instead, Deuteronomy has a strong theory of land-justice. The land will allow residents to remain as long as they practice justice toward neighbors. But when the residents – be they Israelites or others – practice wickedness, God will send another people to remove them from the land.
The Amorites and Canaanites will be the targets of conquest. The Edomites, Moabites, and Ammonites will not, at least not yet.