Courage is super-hard, whether you're acting or resisting. Robert E. Lee Prewitt is courageous precisely because he won't be forced into boxing—he won't be bullied. He's stubborn, but for principled reasons—he wants to be true to himself and to his experiences.
At the end of From Here To Eternity we also see Sgt. Warden express courage, but he does it specifically by doing something. He takes charge during the surprise attack on Pearl Harbor, directing anti-aircraft fire and putting his own life at risk.
Prewitt exemplifies Andrew Jackson's quote, "One man with courage makes a majority."
At the end of the movie, Prewitt and Warden both let their love and devotion to the army carry them into heroism, for the sake of their comrades. This shows a form of courage that benefits the whole.
The guys in From Here To Eternity are unyieldingly committed to duty, despite whatever fate throws at them. Prewitt won't hate the army, no matter how poorly and unfairly his superiors treat him. In fact, he continues to love the army with unswerving devotion. At the end, even though he's suffered a knife wound, he staggers back to his unit to help them resist the Japanese air force—getting shot in the process.
Warden's conception of his duty is pretty intense too. Even though he despises Captain Holmes, he continues to serve him—but he also starts to honor his duty to his men more, acting more kindly towards Prewitt.
Prewitt and Warden become heroic because they remain true to their duty as soldiers. Captain Holmes, his sergeants, and Sgt. Judson become villainous because they fail that duty.
Holmes and Co. are villainous because they do stick to their sense of duty…it's just that their sense of duty is narrow and misplaced. Prewitt is embracing a higher kind of duty—the duty he feels towards his principles, his country, and himself.
In From Here To Eternity, love runs in two directions—towards people and towards higher causes (which also have living people behind them). Prewitt and Alma fall in love, and so do Karen and Warden.
But Prewitt and Warden are also in love with the army, which proves to be their most lasting attachment. They both risk life and limb for it, and for Prewitt, it costs him his life. A good part of the movie's dramatic tension comes from this conflict between personal and patriotic love.
In From Here To Eternity, love is a constructive force. It keeps people knitted together and committed to each other.
In From Here To Eternity, love is destructive. It seems to cause endless difficulties for Prewitt, Alma, Warden, and Karen.
The only marriage we really see in From Here to Eternity is a totally terrible one: the marriage between Dana and Karen Holmes. Dana is an adulterer, a bully, and a total creep, and their marriage is so loveless that they're both openly looking for other people.
The marriages that are actually good are only those that might have been, like a marriage between Karen and Warden or between Prewitt and Lorene. As it stands, the movie tends to investigate "the woe that is in marriage" rather than any potential happiness.
You can view From Here to Eternity as an argument against using marriage as a pointless trap. Karen and Holmes remain married yet completely unfaithful to each other—their marriage is an utterly hollow shell.
On the other hand, you can view From Here to Eternity as an argument for marriage—just not terrible ones.
When it comes to status, it's boys against girls in From Here to Eternity. Alma assigns status a lot of importance—she doesn't want to lead a financially insecure and impoverished life. The fact that Alma wants to rise in the world complicates her relations with Prewitt, and the fact that Warden won't leave his own status behind (albeit for a higher one) messes up his relationship with Karen. Karen doesn't care about Warden's status—but she can't marry him unless he becomes an officer. It's the only way he can transfer out of Holmes' unit.
From Here to Eternity critiques the way social status dominates our lives. The reason Alma and Prewitt, and also Karen and Warden, can't be together happily is purely because of the dictates of social divisions.
Or, you could see the movie as making a practical argument about the role social status plays in life. It's not so much criticizing it as explaining how to roll with it.
We don't see much actual warfare until the very end of the movie— what we mostly see in From Here to Eternity is the lives of the soldiers themselves: the way they spend their down time and the way their daily routine is structured. The movie is a detailed study of that life, in many ways.
We see a soldier's life without warfare—the problems of love, the insane difficulties imposed by idiotic officers, the bar-fights and shenanigans—and then we see life with warfare, when the moment of truth comes. Both Warden and Prewitt are defined by the way they respond to this moment, and they both meet it with courage.
From Here to Eternity neither approves of nor condemns warfare: it just shows what the lives of soldiers are like in peacetime, and how war ends up affecting those lives.
From Here to Eternity presents an argument for war. If you look at the lives the soldiers were leading before the war, they were sort of unconstructive. But when the war starts, the courage of Warden and Prewitt comes to the forefront—they're able to do what they should be doing instead of suffering comparatively trivial annoyances and problems.