Despite what was assumed by the humans before they conducted Operation: FIRST STRIKE, the Unyielding Hierophant was in fact not preparing for an invasion of Earth against humanity, but rather to secure the portal to the Ark, which can only be accessed from Earth. The surviving ships from the Hierophant's destruction are the ships invading Earth at the beginning of Halo 2. Because the ships were not aware of humanity's overwhelming presence they were caught off guard, and consequentially the Prophet of Regret had to make a slipspace jump to Installation 05. Copied from Uneven Elephant's article on the halo wiki.