From what I read, it was not like they let Reach fall just to do the operation. Only the Pillar of Autumn took part in the operation and it was never carried out, fully. The closest the operation got to success was the destruction of the Unyielding Hierophant. This great success was one of the two reasons why the fleet that attacked earth was around a third of what attacked Reach. To sum it up either way, I believe Reach would have fallen either way. Edit: Left out a preposition. [Edited on 01.02.2013 1:12 AM PST]