Innumerable force of Spirits armed,
That durst dislike his reign, and me preferring,
His utmost power with adverse power opposed
In dubious battle on the plains of Heaven
And shook his throne. What though the field be lost?
All is not lost—the unconquerable will,
And study of revenge, immortal hate,
And courage never to submit or yield;
And what is else not to be overcome?
John Milton, Paradise Lost (I.101-109)
What's up with the epigraph?
Steinbeck opens his work with a passage from John Milton's Paradise Lost. It's the moment when Satan and his minions have just recovered from their epic fall from Heaven, and Satan is giving a fairly convincing pep talk to the stunned troops.
Satan describes the rebellion against God and makes sure to let us know that he and his forces at least made a little dent in the Almighty's peace and quiet ("and shook his throne"). But Satan calls this fight a "dubious battle." There's never a question that the Man Upstairs is going to win this one, so what, exactly, is in doubt here?
On one hand, Satan has neither the right to make a claim to the throne of Heaven nor the strength to back it up (they don't call God "The Almighty" for nothing). There should never have been a battle between two such uneven powers in the first place. Dubious, indeed.
On the other hand, Satan would have his minions (and us) believe that he gave God a good scare with his own army. He is full of bravado and self-importance, feeling certain that the outcome of that battle on the "plains of Heaven" could have gone his way.
In the end, Satan rallies himself and his troops from their new, harsh reality by revising the story of the Fall to showcase his "admirable" qualities: courage, strength of will, and perseverance in hate. He does such a good job that we literally feel sympathy for the devil.
So, how does all of this apply to Steinbeck's novel? Who is the devil in this scenario, and who is the conquering God?
Milton gives us a complex Satan, a guy with an unconquerable fighting spirit pitted against the oppressive hand of the most worshipped being in the universe. He's also fighting at a disadvantage, and he winds up consigned to Hell (it's really bad there, folks). We could easily see how Steinbeck might have been thinking of his workers when he saw this Satan.
Milton's Satan also knows how to talk. He has zero problem manipulating the disgruntled angels into believing they had just cause for rebellion. This is not unlike Mac and Jim in the orchards. They'd like London and the workers to believe that it's their fight—not Mac and Jim's. But all the while, we see Mac playing to the men's anger, riling them up to action. We never really know whose fight it is.
Yet it's hard to assign Miltonic roles to the characters in Steinbeck's novel. While we could cast the workers (or Mac) as the proud and defiant Satan, we know that Steinbeck doesn't see them as evil. And since he has no good opinion of the Growers, Steinbeck wouldn't bestow upon them the strange nobility of Milton's Prince o' Darkness, either.
It seems then that the epigraph gives us a general framework to think about the strife in the novel. The battle in Heaven offers us with an archetypal battle between good and evil, one that is also unevenly matched in terms of power—just like the battle in the novel.
Steinbeck seems drawn by that title phrase precisely because it questions why in the world such a calamity should ever have taken place. A more precise reading than this is certainly yours for the creating, but we think there's no need to shoehorn the work too hard into a strict metaphor.