Within Peter Jackson’s film adaptations of The Hobbit and the Lord of the Rings, Sauron is the only dark lord that fans know. Sauron and the Necromancer are the same character, meaning that both of the film trilogies touch upon what happens during Sauron’s bid for power, and his regaining strength behind the gates of Mordor whilst he searches for the ring of power to achieve his total dominion over Middle Earth. In all the portrayals of him, Sauron is a power-hungry, greedy, and completely self-serving being, but it wasn’t always like that.
In the earlier ages of Middle Earth, Sauron served beneath Morgoth, who was the previous ruler of evil before him. And Sauron served him fiercely and loyally, carrying out his every command, battling for the Silmarils at his behest, and only fleeing from his side once Morgoth had been defeated. In the aftermath of the loss of his master, Sauron had to regroup and come up with a strategy moving forward. But many fans question if this had secretly been a part of his plan all along?
Shortly after the fall of Morgoth, Sauron staged an attack against the elves, and it is unclear whether this was a premeditated tactical move concocted before Morgoth’s fall, or if this was an act of wrath and revenge on behalf of Sauron. To many, it seems too convenient that Sauron made a bid for power so soon after the fall of his master, and they wonder if he hadn’t already been planning to usurp and overthrow Morgoth himself. This would fit in with all of the examples of his behavior that audiences have seen from the films, because it is never suggested that he has a greater reason or purpose for starting the War of the Ring than simply wanting to be the unattested dominator of the world, and rebuild it in his own foul image. However, those fans who have read The Silmarillion may suggest a different side to the story.
More die-hard fans of Tolkien's original writing might suggest that Sauron never believed anyone would be able to overthrow Morgoth, as he had stood so uncontested for so long upon the throne or darkness, and had been the driving force of evil for thousands of years before his fall. When the Host of Valinor charged in the War of Wrath, Melkor expended so much of his fea (his soul, the source of his strength and malignant magic) in trying to control the will of others, that he ultimately weakened himself enough to be defeated. This would have come as a great shock to Sauron, who couldn’t possibly have conceived of his impenetrable master losing. Sauron then chose to take up the mantle of Morgoth and became his successor, planning to complete Melkor’s mission to take over.
Once Sauron had a taste of power, his malice only grew, and he was able to add his own fea into the mix during the creation of the rings of power. There is a dark history behind how Sauron managed to trick Celebrimbor into creating the rings that would later enslave the nine lords of men to Ringwraiths, destroy many of the dwarven kingdoms in dragon fire, and ultimately allow Sauron to endure and rise again years later despite the loss of his physical presence during both the fall of Numenor and the Battle of the Last Alliance.
This was all devised by him, but most feel that if Morgoth had suddenly resurfaced at this point, Sauron would have welcomed him as an ally and served alongside him loyally as he had always done, rather than seeing him as an opponent in the contention of supreme dominion over the kingdoms, realms and people of Middle Earth.
Luckily for the fellowship tasked with the seemingly hopeless task of destroying the ring in Mount Doom, Morgoth is unable to ever rise to power again, because his fea was spent, unlike Sauron who wisely locked his away in the master ring. So when Morgoth’s physical body was beheaded, his fea, his spirit, was able to be captured easily and sent by the Valar out into the Void beyond the edges of Middle Earth, meaning that it is eternally trapped in time and space and would never be able to return to Arda.
The combined efforts of several kingdoms, including Gondor and Rohan among many others, was barely enough to defeat Sauron, and many valiant warriors and leaders lost their lives in the attempt, so it would have been even more impossible and hopeless to have any chance of winning the war if Morgoth and Sauron were able to join forces at full power and strike back against the allies.