The Expeditionary Fleets were the great armadas that carried the Great Crusade outward from Terra, mobile war-hosts that swept from system to system bringing world after world into compliance with the nascent Imperium. Each was a self-contained instrument of conquest and governance, numbered in the hundreds, bearing Legion companies, Imperial Army regiments, Mechanicum cohorts, and the machinery of an entire civilisation on the march.
More than mere fleets of war, they were engines of Imperial order, carrying iterators to preach the Imperial Truth, remembrancers to chronicle the age, and administrators to bind newly-won worlds into the growing dominion. A single fleet might range for decades far beyond the light of Terra, its commanders wielding near-absolute authority over the campaigns they prosecuted.
Legions such as the Ultramarines and the Space Wolves commanded their own vast expeditionary forces, and it was through these scattered, far-flung fleets that Horus spread his treachery. When the Heresy came, distance and isolation ensured that loyal and traitor forces alike were caught unready across a thousand lonely stars.