Well they would head for Iseangard and Saruman would not be prepared so I think they would wipe them out with calvery. And Gandalf would take care of Saruman. There would not be much to the battle of Isengard with just men and no other forces to oppose them.
But Gandalf was easily able to disarm Saruman when they came after the Ents attacked, why didn't Saruman have any power then? I think it would've taken a bit longer and at a greater cost of life, but I believe that an army led by Gandalf the White could've taken Isengard.
Yes, Gandalf the White could have killed Saruman. Because he was higher in counsel. He can in a way over throw Saruman. So yes Army Led by Gandalf the White would provale. But just men from Rohan, they would stand no chance.
But you didn't say with Gandalf. And by the way why do you think Rohan would win against the Uruk-Hai. Helms Deep was a tough battle. And Rohan had the advantage.
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