Israel was to undertake war only with the purpose of dispossessing former inhabitants of the area the Lord had promised them; Edom was not part of that area, and so the Lord forbade his people from engaging them in war. He had instead given Edom to Esau for a possession. God held power over peoples and nations in addition to Israel.
5 Do not contend with them, for I will not give you any of their land, no, not so much as for the sole of the foot to tread on, because I have given Mount Seir to Esau as a possession.