It's not just about having the power...

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Originally Posted by Legate of Amon Lanc

However, like I said, I think there was no real confrontation between Gandalf and Saruman. When the negotiations failed, Gandalf stopped, he did not attack - but at the same he did not capitulate, did not submit, did not do what Saruman wanted from him. I believe Gandalf didn't try to use power to contest Saruman there. In PJ's movie, the one who starts the "duel" is Saruman, because he becomes angry. But in the books, there was no reason for him to become angry to attack Gandalf - it says he was "cold and perilous". He was "cold", he was aware of his superiority at the moment, so he just sent the guards to escort Gandalf to the top. Saruman wouldn't have started anything. So, the only one who could have tried to use his power to get away would be Gandalf then. Gandalf the aggressor? And that's what I believe he won't do.

While I agree there was no wizard duel between the two I don't think that Gandalf allowed Saruman to imprison him because he thought it was the right thing to do. The way I see it he simply did not have a choice, not because Saruman was mightier than him, but rather because he was alone in the fastness of Orthanc, surrounded by thousands of armed servants of Saruman. Gandalf the Grey was indeed a deadly foe (and at times the aggressor too, at least against Orcs) but he could not take out armies all by himself.