Yeah, Sauron got beaten with the Ring.
... by NUMENOREANS.
So Sauron had some magic metal, so what? That match up is like Super Saiyan 3 Goku versus some guy with boxing gloves.
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Anyway he was going to win in the Third Age because of the decline of Men due to infighting and remnants of his Second Age army beating up on them all the time (see Gladden Fields). As the return of Men from Numenor was relatively recent at the time of the Last Alliance they still wielded enough power to defeat him. I doubt they could have done it twice though, as they only barely won even then. And as I remember Sauron made war too hastily after the fall of Numenor, before his forces were full strength.