It's not just about having the power...

[Legate of Amon Lanc]

Originally Posted by Morthoron

Actually, Elendil and Gil-galad won. Isildur plucked up the ring after the battle was over.

Well, of course - although for sure you are not going to deny him his part... The point was, that he was the (last remaining) representative of the victorious forces (of Men, resp. Dúnedain) and his enemy was defeated, yet he kept the Ring (and like everybody else, he found himself a good starting reason why to keep it. "For our beloved King and my poor brother who died, we should keep the Ring." And later, in the hands of a corrupt king, it could become "In the memory of our beloved King, let us take revenge upon these and these..." Sounds all too familiar excuse.)