Could Gandalf have won the war alone?

[Boromir88]

Now, true enough, the great weakening could simply referr to the loss of increased power the ring gave.

I think it would refer to him losing his Ring, as in the Letter to Milton Waldman Tolkien does say the Ring enhanced Sauron's power.

One more thing I'd like to point out is, I think with each time Sauron was killed and had to reform, he lost more of his power:

'But that of course did not destroy the spirit, nor dismiss it from the world to which it was bound until the end. After the battle with Gilgalad and Elendil, Sauron took a long while to re-build, longer than he had done after the Downfall of Numenor (I suppose because each building-up used up some of the inherent energy of the spirit, which might be called the 'will' or the effective link between the indestructible mind and being and the realization of its imagination).'~Letter 200

Technically if you could just kill Sauron enough times, he wouldn't have the will power anymore to reform. As each time Sauron 'rebuilds' or has to rebuild, it takes him longer and I got a sense that he gets weaker as he loses his 'inherent energy' or as Tolkien calls it 'the will.'

By the War of the Ring, I think Sauron had enough force (militarily) to beat down the West, so defeating Sauron by force wasn't an option. However, each time Sauron died and had to reconstruct a new body, he appears to have been weaker than the previous.