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In all honesty, this whole Dol Gúldur episode makes little sense, army or no army. I'm leaning towards the army alternative though (Celeborn and Elrond's men, supported by Saruman's devises, whatever those may be) as to me it's more plausable. If there were no army, are we then to believe that Galadriel, Gandalf, Saruman and Elrond trekked across Middle Earth all by themselves to oust Sauron the Great from his feared stronghold, no doubt defended by Orcs and Eru knows what? That seems a tall order to me.

Even if it were so and they managed to drive Sauron out (or at least, believing it), how could this be possible without actually conquering the fortress? A few wizards and Elves, albeit very powerful, can hardly besiege and starve out the fortress. Why would Sauron run (or: how could he successfully feign to run) if he was safe behind the walls of his Stronghold? And why wait with driving him out if they thought they could manage it alone without much effort or bloodshed (which they must have if they went there alone)? The main reason for waiting would be not wanting to fight a hard battle incurring heavy casualties no matter the outcome if it wasn't absolutely necessary.