At the height of its power,
Gondor dominated the territory that had been
Mordor. I suppose Gondor at the height of its power surpassed that which it possessed in alliance with
Arnor in the Last Alliance, but of course, the
Elves were no longer in alliance with them. And while Gondor was able to defeat the combined
armies of
Minas Morgul and the
Southrons on the Pelennor Field in alliance with
Rohan and the army
Aragorn brought from Pelargir (the
Easterlings that beset the
North Road in Anórien did not enter the battle, fortunately), it was still a near-run thing until Aragorn arrived.
Gandalf observed in “The Last Debate” that, “Hardly has our strength sufficed to beat off the first great assault. The next will be greater.”
You must also reckon that Ciryaher Hyarmendacil I ruled from
Osgiliath, which was still a populous
city, probably at its height as well, and controlled
Minas Ithil, which would not fall for another thousand years, as well as South Gondor, Near
Harad, and
Umbar, as well as the territory as far north as the Undeeps north of the
Argonath and all of what was later the Wold of Rohan south of the Limlight. Gondor’s power extended along the Ered Lithui north of Mordor, it controlled the
Towers of the Teeth,
Cirith Ungol, and even “
the old castle of Durthang” (
Return of the King, “The Land of Shadow”: the old castle-become-Orc-hold
Sam and
Frodo saw before they were overtaken by the
Orcs and their slave-drivers) at the north end of the Ephel Dúath west of
Udûn where the Ephel Dúath and the Ered Lithui met. Gondor dominated the region of
Rhovanion and east to the far side of the
Sea of Rhûn.
Sauron was ensconced in
Dol Guldur maintaining a low profile.
By the end of
the Third Age, Gondor’s territory, even in alliance with Rohan, was greatly reduced, perhaps to a third of less of its former
realm, maybe even a quarter. “[M]uch of the best blood in Gondor was shed” at the battle of the Crossings of Erui during the Kin-Strife of 1432-1448, at the end of which the followers of Castamir the Usurper established themselves
with the fleet of Gondor(!) at Umbar, consisting not only of much of the merchant fleet of Gondor and nearly all its naval power, but of a great many noble and well-trained folk of Gondor, much weakening the kingdom.
Could Byzantium at its height, around AD 600, have defeated the Ottoman Caliphate of 1453? By 1453, Constantinople had lost nearly all its territory, its resources, its people and numbers, yet it still put forth a mighty effort in the siege that led to the ruin of Byzantium and the fall of Constantine XI Palaiologos. But these are apples-to-oranges comparisons: Neither the Ottoman Turks could approach Constantinople at its height, nor the armies of Sauron approach
Minas Tirith at the height of Osgiliath’s power when
Minas Anor was but a protective
fortress. Time, place, and circumstances determine much of the outcome of battle and war.
Consider these: had Napoleon not suffered hemorrhoids so that his attack was delayed by hours, would he still have lost to Wellington and Blücher at Waterloo? Had J.E.B. Stuart been at hand when needed, had Stonewall Jackson not been wounded and still alive and in command, had Robert E Lee not suffered a heart attack, would the Confederacy still have been defeated by Meade at Gettysburg? Had the field at Agincourt not been soaking wet, would Henry V still have defeated the Dauphin and his enormous army?