The immediate context: Isaiah continues the thought of the coming judgement against the Assyrians. But in the larger context, we can see this is an announcement concerning the judgment of teh nations during the Great Tribulation.
Edom was a neighboring nation to Israel and a bitter rival. 2 Kings 16:6 records an account then the king of Aram and the king of Israel tried to overpower the king of Judah but was unable to do so but states "Rezin king of ram recovered Elath for Aram by driving out the people of Judah. Edomites then moved into Elath and have lived there to this day." Edom hated Israel more than any other nation. Edom is frequently used as a representtive of all nations that hated the Jews.