It is thus probably to Sauron that we may look for a solution of the problem of chronology [of when the Orcs were bred]. Though of immensely smaller native power than his Master, he remained less corrupt, cooler and more capable of calculation. At least in the Elder Days, and before he was bereft of his lord and fell into the folly of imitating him, and endeavouring to become himself supreme Lord of Middle-earth. While Morgoth still stood, Sauron did not seek his own supremacy, but worked and schemed for another, desiring the triumph of Melkor, whom in the beginning he had adored. He thus was often able to achieve things, first conceived by Melkor, which his master did not or could not complete in the furious haste of his malice.
— Tolkien, J. R. R. The History of Middle-Earth X: Morgoth’s Ring. Ed. Christopher Tolkien. (London: HarperCollins, 2002.) 420. (Myths Transformed, Text X, “Orcs”