The Power of Isengard

[Tuor of Gondolin]

Not necessarily. There is one crucial favorable geographical feature for the good
guys, the White Mountains. Assuming a fairly evenly divided bad guys (with
Saruman holding the ring but unsure of how to effectively control it) Rohan can
withdraw to mountain strongholds and across the Isen, and Gondor to South Gondor. If they beat off the corsairs there could even be effective coordination to the west between Gondor and Rohan, and Mordor forces would be
stretched between defending Mordor, threatening the good guys and attacking
Saruman. It could be a long, nasty war of attrition- especially if there was some
peripheral Lorien intervention and Saruman/Sauron fighting spread to the fringes
of Fangorn and the ents felt it necessary to intervene.