 Halo 3 is set in a science fiction setting during the years 2552 and 2553 . In the year 2525 , a theocratic alliance of alien races known as the Covenant discover humanity spread across dozens of interstellar colonies . The Covenant declare humanity an affront to their gods and begin destroying colonies by bombarding the planets with barrages of plasma , turning their surfaces into glass . Despite efforts to keep the Covenant from finding further human worlds , a Covenant fleet discovers Earth during Halo 2 . 
