Since you mentioned
Battle of the Pelennor Fields I take with
Sauron's victory there much of the strength of the West would have been lost. I take the losses would be:
1.
Theoden,
Eomer, Marshalls of
Rohan and most (if not all) of the rest of the 6,000
Rohirrim.
2. Prince
Imrahil of
Dol Amroth and his Knights.
3.
Faramir and the
Guard of the Citadel.
4. Rest of the remaining inhabitants and defenders of
Minas Tirith.
Additionally
Gandalf would have been killed as well if he could not flee. It is likely he would have chosen to make a last stand, perhaps against the
Nazgul, rather than flee. Plus if
Aragorn,
Legolas,
Gimli,
Rangers and the rest of Gondrians from the South did still arrive they could not have been able to overcome the With-Kings Army.
Aftermath of the Fall of Minas Tirith:
- In Rohan few riders and leaders remained to defend it. In the face of an impending invasion from
Mordor and her allies the Rohirrim would have to make a last stand at the strongholds of Edoras, Dunharrow,
Isengard and
Hornburg. Of them the latter two were heavily damaged.
Fangorn Forest would not stand much of a chance either.
- In
Gondor supposing Aragorn and Co. survived they would retreat to Pelargir. Since the Corsairs were defeated Sauron would have to assign newer
armies to the conquest of Gondor, that would take some time. Therefore it would only be a matter of time before the weakened, manpower lacking
cities/towns of Pelargir, Dol Amroth, Edhellond, Calembel, Ethring, Tarnost and Linhir fell and the destruction of Gondor was complete.
- The Siege of
Erebor maintained, morale of the defenders plummet while that of the besieging army rise. Possibly Erebor would soon be breached.
-
Lorien and
Mirkwood Elves continue to fight, maintaining only a defence of their
realms. Mirkwood would likely fall earlier since Thrandiul did not have a
Ring of Power while
Galadriel did.
-
Beornings and Woodmen continue to fight and suffer losses until they are eventually overcome.
- When Lorien will fall next to be attacked would be
Rivendell.
Now despite the fall of Gondor and Rohan
Frodo and
Sam would most probably still have managed to destroy
the One Ring. Thus Sauron would still be defeated despite acquiring a virtually complete conquest of
Middle-Earth.
In the actual story the
Easterlings and Haradhrim continued to fight on even after the end of Sauron and were only subdued when Rohan and Gondor invaded
Rhun and
Harad. Therefore if Sauron had managed to destroy Gondor and Rohan forever there would have been nothing to stop the Easterlings and
Southrons from marching all the way to the extreme West of Middle-Earth; that is
the Shire,
Blue Mountains and
Lindon.