[William Cloud Hicklin]

It seems that Sauron's older plan was a "northern strategy", but it hinged in great part on making use of Smaug. Bilbo, Thorin and Bard kind of scotched that idea; and Sauron's principal base of operations was moved from Mirkwood south to Barad-dur thanks to the White Council.

Tolkien was also at least generally aware of logistics- the ease of an operation depends enormously on the length of one's supply lines. Minas Tirith was closest to Mordor (and the supposed capital of the new Ringlord).

Rivendell was a long way away and on the other side of the Mountains, and so pretty much unapproachable so long as Lorien stood and the Gap of Rohan held, and Gondor prevented his "marching in power along the coasts" as Boromir put it. Sauron thought he was dealing with each of these problems. He assaulted Lorien (apparently from Dol Guldur): this army was repulsed and deflected into northern Rohan, where the Ents destroyed it.