Gandalf, Moria, and Arrest

[Moonraker]

Originally Posted by Belegorn

Yes, but even if this is so, what has this to do with Rohan?

Rohan could have held Boromir in custody with the agreement of Theoden once the crime was explained. But that was never going to happen, as the Ring was mean't to be kept secret from Rohan, and the Company would not have risked going to Edoras with Isengard marshalling an army close by. Also, Denethor would have pleaded with Rohan to have Bormomir pardoned and released at once.