Q: When Was Sauron at His Most Powerful?
ANSWER: There is no straight forward answer to this question because J.R.R. Tolkien made two explicit remarks about the “strength” of Sauron at different times in Sauron’s career. For example, in a footnote attached to Letter No. 183 Tolkien wrote:
When he found how greatly his knowledge was admired by all other rational creatures and how easy it was to influence them, his pride became boundless. By the end of the Second Age he assumed the position of Morgoth’s representative. By the end of the Third Age (though actually much weaker than before) he claimed to be Morgoth returned.
In the essay Christopher Tolkien titled “Notes on motives in the Silmarillion”, published in Morgoth’s Ring, J.R.R. Tolkien wrote:
Sauron was ‘greater’, effectively, in the Second Age than Morgoth at the end of the First. Why? Because, though he was far smaller by natural stature, he had not yet fallen so low. Eventually he also squandered his power (of being) in the endeavour to gain control of others. But he was not obliged to expend so much of himself. To gain domination over Arda, Morgoth had let most of his being pass into the physical constituents of the Earth – hence all things that were born on Earth and lived on and by it, beasts or plants or incarnate spirits, were liable to be ‘stained’. Morgoth at the time of the War of the Jewels had become permanently ‘incarnate’: for this reason he was afraid, and waged the war almost entirely by means of devices, or of subordinates and dominated creatures. Sauron, however, inherited the ‘corruption’ of Arda, and only spent his (much more limited) power on the Rings; for it was the creatures of earth, in their minds and wills, that he desired to dominate….
According to the essay just cited, Melkor/Morgoth was at his most powerful at the very beginning of the world (Arda) because he had not yet diffused his own essence into anything. Sauron, on the other hand, created the One Ring in order to enhance his personal power. It is therefore arguable that he did not achieve his greatest personal power until the middle of the Second Age. But Sauron’s military power during the Second Age was never sufficient to conquer Middle-earth. The Eldar and Numenoreans defeated him in Second Age year 1701; and Ar-Pharazôn’s first armada was large enough that Sauron’s armies and allies fled away rather than face the Numenoreans in battle. And at the end of the Second Age the armies of the Last Alliance of Elves and Men were able to defeat Sauron’s armies.
In the Third Age Sauron implemented a policy of dividing his enemies and conquering or weakening them gradually while he built up his own forces. Thus at the end of the Third Age the Free Peoples had no hope of defeating Sauron by military action. Their only hope was to destroy the One Ring of Power and unravel Sauron’s control over the numerous tribes and nations that served him. Thus militarily Sauron was more powerful at the end of the Third Age than he had been at the end of the Second Age, but he was personally more powerful at the end of the Second Age than he was at the end of the Third Age.