Actually the skills of the Dunedain and Elves would probably almost make armor a requirement if Sauron ever planned to be on the battlefield. After all, how could he be sure that his enemies would be so noble as to keep themselves confined to close combat when he was concerned. Up close and personal, he had is power and his fire. But what would happen if they had decided to take him out from a distance. Both the elves and men possesed in thier armies numerous incredibly crack shots with bow and spear; what's to keep them from simply taking aim and riddling him. I imagine that any fear powers Sauron has won't work on intert objects and using his fire to burn them rather relys on him being able to see all of them. I tend to think his firepower requires conscios effort, that his background heat was not so high that arrows and spears simply spontaneously combusted as they got near him (and even if they did, the heads would still get though which would funtionally be the same as the whole arrow or spear. Plus assuming anyone knew about the fire thing (and they probably did. All they'd need are arrows or spears with iron shafts as well as heads). No armor means no defense against projectiles; Sauron would be a pincusion before he got twenty paces out of the gates. If his finger can be cut off, his body can presumably be peirced.
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