Enemies

[Raynor]

I wouldn't say that Men never fight Men, nor that these mannish enemies are always unredeemably corrupt. Some of the inhabitants of Umbar used to be numenoreans too.

Beside this, another level of drama we witness in Tolkien is that of the deceptions of the Enemy which turn humans against each other or at least against good causes - whether this the lies of Melkor/Sauron turning elves against the valar, men against elves, men against men, men to almost side with Saruman, etc. It seems that these lies are one of the most enduring weapons of evil, with some of the most tragic effects too.

Edit: I also remembered this interesting passage from Myths Transformed, which may be relevant:

...though of necessity, being the fingers of the hand of Morgoth, [orcs] must be fought with the utmost severity, they must not be dealt with in their own terms of cruelty and treachery. Captives must not be tormented, not even to discover information for the defence of the homes of Elves and Men. If any Orcs surrendered and asked for mercy, they must be granted it, even at a cost.* This was the teaching of the Wise, though in the horror of the War it was not always heeded.

* Few Orcs ever did so in the Elder Days, and at no time would any Orc treat with any Elf. For one thing Morgoth had achieved was to convince the Orcs beyond refutation that the Elves were crueller than themselves, taking captives only for 'amusement', or to eat them (as the Orcs would do at need).