Sam's Debate and the Voice of Despair

[Knight of Gondor]

I recently picked up Fellowship of the Ring to verify some fact (I forget now what it was) that was in confusion in my mind from the movie. Naturally, I could not put it down (wish I'd started at the beginning) and am now with Frodo and Sam again at the foot of Mt. Doom. (This is probably the eighth or ninth time I've read it.)

I'm curious about the debate that Sam had with himself. Sam ended it with the viciously loyal note "I'll get there if I leave everything but my bones behind, and I'll carry Mr. Frodo up myself, if it breaks my back and heart. So stop arguing!"

This appears to be a crescendo of an internal argument that took place with Sam on a couple of instances.

My question is, was Sam really just battling his own inner doubt, or was there some external evil at work? I ask mostly because, right at the end of the last debate, the ground shakes and there is a brief flash of light. In most cases in Lord of the Rings, when the weather or elements show some sign, there is external supernatural powers at work. (IE, Meduseld/Theoden's healing, Helm's Deep, Pelennor, etc.) It would seem that Samwise was victorious over this doubt, internal or external, and the enemy suffered a defeat.

What are your thoughts on the matter?