Historians differ about the historical background for the author only gives vague details of this. Perhaps the best guess of when it was written is following the fall of Jerusalem and subsequent exile into Babylon. Although Esau and Jacob ultimately parted ways on good terms, the history for the next 1000+ years records several conflicts between Edom (where Esau and his descendants lived) and Israel (Judah, where the descendants of Jacob lived). Edom no doubt rejoiced over and probably took advantage of the fall of Jerusalem and very possibly had common cause with other neighboring nations that are listed in verses 19-21.
