[The Saucepan Man]

Aragorn clearly doesn't accept Denethor's annulment of the oath

Legally, it might be said that Denethor was not of sound mind, and so his annulment of the oath was ineffective.

Finally, what does it say about Aragorn that he swears an oath of service to Frodo : ' I am Aragorn, son of Arathorn; & if by life or death I can save you, I will' when he's only just met him?

Does Aragorn pledge his service to Frodo as an individual or to Frodo as Ringbearer? If the latter, then his duty is to prevent the Ring falling into Sauron's clutches. However, I take your point, davem, that he pledges to save Frodo and not the Ring.

If his pledge is to Frodo personally, then arguably he breaks it at Parth Galen when he lets Frodo go without following him. On the other hand, if his duty is to the Quest, then he might be said to be discharging his duty if he concludes that Frodo and Sam have a greater chance of succeeding on their own.

This raises another issue (and ties in with my earlier point about Merry and Pippin acting in the interests of the realm to which they have sworn fealty by disobeying the individuals to whom they pledged their service). What scope does an oath-taker have to use their discretion to discharge their duty? If, for example, a character pledges themselves to a particular cause and takes a course of action which they think will further it but which in fact is detrimental to it, can they be said to have broken their pledge? I would say not if they genuinely (albeit mistakenly) believe that what they are doing was in furtherance of their oath.