Could Gandalf have won the war alone?

[Raynor]

Sauron has significantly increased his power, through his own talents and through the forging of the Ring, over the course of two ages of the Earth, so he is more than a match for any number of Maia.

I would disagree with that:

Originally Posted by The shadow of the past, FotR

This is the One Ring that he lost many ages ago, to the great weakening of his power. He greatly desires it – but he must _not_ get it.

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

Originally Posted by Letter #246

One can imagine the scene in which Gandalf, say, was placed in such a position. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors.