Halo: Contact Harvest (Halo #5)

This is how it began... It is the year 2524.  Harvest is a peaceful, prosperous farming colony on the very edge of human-controlled space.  But we have trespassed on holy ground--strayed into the path of an aggressive alien empire known as the Covenant.  What begins as a chance encounter between an alien privateer and a human freighter catapults mankind into a struggle for its very existence. But humanity is also locked in a bitter civil war known as the Insurrection.  So the survival of Harvest's citizens falls to a squad of battle-weary UNSC Marines and their inexperienced colonial militia trainees.  In this unlikely group of heroes, one stands above the rest...a young Marine staff sergeant named Avery Johnson.