Well, Thinlomien, thats a good question. I believe that Sauron (if he knew about the Ring sailing over the sea) would probably have sent the Corsairs after it, and in the meantime the host of Minas Morgul would have sieged and crushed the defense of Minas Tirith. At that, even if the Rohirrim came then to their aid, they also would have been slaughtered by the Haradrim and the remaining orcs. No Aragorn and no Gandalf, and no Army of the Dead to come and save the day!
