Orcs: Second Age (p2)

However Sauron had not been able to gather all the Eastern Orcs into his armies, as they at first scorned him, not realising the fair 'Annatar' was Sauron. It was only after his first defeat that Sauron went among them in dark guise and was thus able to begin rebuilding his power.

Until Sauron's final downfall in the Third Age, Orcs remained the backbone of the armies of Mordor, and of Isengard when Saruman ruled it.