Let's take a second look at that Keats poem in the packet: "On First Looking into Chapman's Homer." Did you spot the mistake? The Conquistador who first "star'd at the Pacific" was Balboa, not Cortez. When he drafted the poem in 1816, Keats had just read a book on the Conquistadors, and blended together in his imagination two separate but related events: Cortez's "discovery" of the Valley of Mexico and Balboa's sighting of the Pacific from "a peak in Darien" (modern-day Panama). In fact, a friend of Keats immediately spotted the mistake when he saw the poem in manuscript; interestingly, Keats decided to leave the line as it was. For one thing, "Balboa" would have disrupted the sonnet's iambic pentameter beat by adding an extra syllable. Perhaps Keats really like the phrase "stout Cortez" (so much better than, say, "rocky Balboa"). What matters here is that poetic truth trumped historical truth--a deeply Romantic gesture: the triumph, you could say, of the imagination over the "mind-forg'd manacles" (Blake) of the merely factual.
It's appropriate then that this creative "mistake" occurs in the midst of a brilliant comparison (what literary critics would call a conceit) that "illumine[s]" (Shelley) in our minds this moment of discovery through the efforts of an artist (in this case, Chapman the translator) of a new world, the world of Homer. It's a world of the imagination (Homer's), for the imagination (Keats the reader), and--communicated--by the imagination (Chapman). In other words, imagination is sovereign. Indeed, the imaginative intelligence is the sovereign ("That deep-brow'd Homer ruled as his demesne").
Now, I don't think that the Romantics believed that a poet should rule over a literal kingdom. They were children of the Enlightenment (though some members of that first generation I mentioned outlived the fiery second generation and become rather reactionary old men). They would have applauded Madison assertion in Federalist # 49 that "the people are the only legitimate fountain of power" (Kammen 198). But the Romantics believed they had a mission to help get the people, in whom sovereignty resides, to a better place, a better world. That task, for them, was part of the vocation of being a poet. As Shelley say in The Defence of Poetry: "Poets are the unacknowledged legislators of the world." They are part politician, part priest. Their job is, oracle-like, to "apprehend" new oceans of possibility, a "new planet" (Keats) fit for human living, a New Jerusalem, in which "midnight streets" (Blake), "muddy spring[s]," (Shelley), and "dark Satanic Mills" (Blake) will be seen no more.
Where, you might ask, does this audacious plan, in the preface to Milton, come from, this extraordinary ambition to not just improve society and liberate those "charter'd street[s] and hemmed-in river, but also to "build Jerusalem, / In Englands green & pleasant land"? Well, keen students of last semester's reading will pick up on the imagery and the energy of the Book of Revelation. But Blake was also inspired by a non-Scriptual source. There is a story, long cherished in England, that in the missing years of Christ (between twelve and about twenty-eight, of which the Gospels say nothing--what Dr. Estess would call a notable "narrative silence"), the boy Jesus was brought to England by Joseph of Arimathea, a merchant who went to Cornwall to buy tin, as the Phoenicians did. Hence the prospect, in Blake's poem, of "the Countenance Divine, / Shin[ing] forth upon [England's] clouded hills" long, long before the Industrial Revolution had blighted the landscape. A far-fetched notion, you might say, mere wishful thinking. But also, perhaps, another brilliant mistake.