How will the ring save the free people?

[Alcuin]

Boromir understood the Ring quite sufficiently to comprehend – probably correctly – what he could accomplish with the Ring in “The Breaking of the Fellowship” in FotR:

It is mad not to use it, to use the power of the Enemy against him. The fearless, the ruthless, these alone will achieve victory. What could not a warrior do in this hour, a great leader? What could not Aragorn do? Or if he refuses, why not Boromir? The Ring would give me power of Command. How I would drive the hosts of Mordor, and all men would flock to my banner!

Gandalf told Frodo that “The Ring had given [Gollum] power according to his stature.” And when Frodo asked her why he could not see all the bearers of the Rings of Power and know their thoughts, Galadriel replied, “Did not Gandalf tell you that the rings give power according to the measure of each possessor? Before you could use that power you would need to become far stronger, and to train your will to the domination of others.” Besides the observation that she was remarkably well-informed about a private conversation, Galadriel reinforced what Gandalf said: the more powerful its bearer, the greater the power of the Ring. In his Letters, Tolkien remarked that only a creature of equal stature with Sauron could withhold the Ring from him: Aragorn could not, nor could even Elrond or Galadriel, though they’d have put up quite the fight! Only Gandalf (and presumably Saruman) could have used the Ring as Sauron would, to its full effect.

But Boromir was undoubtedly correct: he’d have become a commander of tremendous power using the Ring; just not powerful enough to defeat Sauron and keep the Ring from him forever.