The True Master.

[Nilpaurion Felagund]

Sauron, without his Ring, is almost nothing. (Gurthang)

You might also say that I, without my internal organs, am nothing.

The Ring is a part of his existence, so he needs it. But not with him, exactly. Just as long as its there. (Of course the safest place it would be is right in his finger, so he would try to find it.)

As long as the the Ring exists, Sauron's will exists. That's why if someone used its powers, he would in the end replace Sauron. Sauron would still exist, albeit in a different form--like how Morgoth's will is still in Arda, despite him being without it. Thus many mini-Morgoth spring out of Middle-earth.

So you might call the relationship between Sauron and the Ring egotism, in a weird way.

All Sauron needed was for the Ring to continue to exist. (davem)

Exactly. Well, better said.

If Gondor had been alert, collected armies from when the first shadow fell, had it been of the might in the days of its rising, had the line of kings not been broken due to human foolishness, would Sauron have such advantage. No. (daeron)

I should just point out that the decline of the Númenóreans as a race is not due to anything foolish Men had done:

[T]he waning [of the Dúnedain] . . . proceeded, little by little . . . For no doubt it was due above all to Middle-earth itself, and to the slow withdrawing of the gifts of the Númenóreans after the downfall of the Land of the Star.

LR Appendix A I iv--emphasis mine

The slow, steady decline of the might of the Dúnedain is a natural thing. So, they could have remained alert and assembled armies and all, but I don't think that, force-on-force, they could have prevailed upon Sauron.

And remember, the Second Age-Númenóreans also had help from Beleriandic-Wars veteran Elves (or their descendants). Gondor had no such heavy-hitting allies, except Rohan (which is, still, relatively small.)

. . . Sauron may have gotten control over the Nine and some of the Seven, but I don't think he did over the Three. It was because Sauron put so much of his will into the One that Celebrimbor knew of his treachery before he was even able to see the Three. (Lhuna)

No, daga. They came from the same blueprint--Sauron's. So they were still under the One.