Edom was a nation traditionally believed to be descended from Jacob’s brother, Esau. In the book, God promises to wipe out the people of Edom for their part in helping a foreign power attack Jerusalem (Team Jacob). Obadiah also prophesies that a restored Israel and Judah will take all their land. In short, when it comes to Esau, no more Mr. Nice God.
This prophecy is commonly believed to refer to the Babylonian captivity of Judah in 586 BCE and the subsequent rebuilding of the temple in Jerusalem.