“However, in the first year of Cyrus king of Babylon, King Cyrus issued a decree to rebuild this house of God.” Ezra 5:13

            A king was considered completely right and never wrong so his decrees had an authority to them nearly equal to God.  Cyrus’ decree still carried some weight that the existing king honored.  Remember, at some time, every existing king will pass and want his decrees followed after his death.  So if one king nullified a previous king’s decree, then it set a precedent for his decrees to be nullified in the future.