Real life causes are not clear cut because human tyrants are seldom utterly corrupted into pure evil will. Even the most corrupt have followers only partly as corrupt, and many leaders still need to have "good motives". In conflicts about important things or ideas Tolkien was most impressed with the importance of being on the right side. If a side has the right, which depended on values and beliefs above and independent of a particular conflict, then it justifies that side's cause throughout.
Tolkien spoke not of individuals. The rightness of a cause will not justify the actions of its supporters who are morally wicked. Aggressors are primarily to blame for evil deeds proceeding from their original violation of justice. They had no right to demand that their assaulted victims should not demand an eye for an eye. Similarly, good actions do not justify the wrong side. One may honour and rejoice at acts heroic courage or deeds of mercy, but it still would not alter a judgment as to which side was right.
His story did not deal with Absolute Evil, said Tolkien, doubting that there is such a thing. No rational being is wholly evil. Sauron was as near an approach to the wholly evil will as is possible. Yet he had begun well, at least on the level of desiring to order all things according to his own wisdom. But he went further than any human tyrant in pride and lust for domination, being in origin an immortal spirit, of the same kind as Gandalf and Saruman but of a far higher order. In The Lord of the Rings the basic conflict was not about "freedom" (though it was involved), but about God and His sole right to divine honour. The Eldar and Númenóreans believed in The One and held worship of any other an abomination.
Sauron desired to be a God-King. He was thrice involved in treachery: First, admiring strength, he had followed Morgoth and become his chief agent in Middle-earth. Second, when Morgoth fell he forsook his allegiance, but from fear only and remained in Middle-earth. Third, seeing how his knowledge was greatly admired and how easy it was to influence other rational creatures, he re-assumed the position of Morgoth's representative in the Second Age. By the end of the Third Age, although actually weaker than before, he claimed, in his pride, to be Morgoth returned. If victorious he would have demanded divine honour and absolute temporal power over all. Even if "the West" had bred or hired orcs or ravaged the lands of other men, their cause would have remained indefeasibly right.
Thus the fiddle-faddle in reviews, said Tolkien, as to whether his "good people" were kind and merciful and gave quarter (which they did) was beside the point. Some critics seemed determined to represent him as a simple-minded adolescent and willfully distorted what was said in his tale. Denethor alone proved this but none of the people on the "right side" were any better than men have been, are, or could be. Middle-earth is not an "imaginary" world but an imaginary historical moment in our habitation
TOLKIEN LETTER#183 Summary
Source: Tolkien gateway