What caused Frodo to finally give in to the power of the Ring and claim it?

Marile,

Excellent points regarding Frodo's desires. I agree with everything. But there is something else, besides Frodo wanting to make his own decisions and protect himself- he also wants to control others directly, to make them follow his will and see the world the way he sees it. A good example of this is his control of Gollum: Frodo just uses him for his own benefit, and clearly enjoys the power of command, enhanced by the Ring. He 'trains his mind to the domination of others', like Galadriel suggested, and by the end he's pretty sure he CAN succeed as the dark lord. It's just in his personality- he's more dominant than submissive, so the Ring for him is a comfortable device to gain power and rule others. That's one of his inner desires.