The Marine Corps motto is "semper fidelis" - always loyal. It's the motto every Marine in Generation Kill is theoretically serving under. Watch closely though and throughout we see how the campaign splits that single word into as many objects of loyalty as there are men holding it. By the end of the invasion, what each of them turns out to be faithful to - the Corps, the mission, each other, their own conscience - that is the real story the show is telling.
Take Brad and Nate, for example.
Brad's fidelity is to his craft. Everything about Brad, whether in the book or on screen, is oriented around mastery. It is all about his precision, his control, him being the best Marine. He summits Mt Shasta on a broken ankle, he tells Evan the only thing he misses from home is his motorcycle "speed, solitude, no one can touch me." His sense of self was built inside a controlled, disciplined system. Based on what we know of his chaotic domestic life - given up for adoption, wayward childhood, a fiancée who betrayed him - it's not hard to see why. The Corps isn't a means to an end for him, it is the point. The Corps is where he found that one arena which appreciates his particular gifts. So even though OIF tests him morally in ways earlier deployments didn't, he stays in.
By the finale he's recovered from his crisis of conscience and looks almost serene over an MRE. He got to keep the thing he's faithful to.
Nate visibly looks drained by the finale because Nate's fidelity is to his conscience.
Nate's whole arc is a slow-build reckoning of how much the institution he serves is broken. Schwetje is a coward, Griego is petty, dangerously so; Godfather only cares for a ribbon. Nate sees the chain of command's willingness to sacrifice lives for optics. And that is a struggle for his conscience to calibrate. Nate never stops being a good officer, but he finds that increasingly, he has to fight the system to be one. By the end, Nate has concluded the Corps is not a place for what he values - intellectual honesty, competence, and care for the men under him. The Corps is a bureaucracy (he sees this is what command is, after all) that forces him to choose between doing the right thing and staying in line.
For Brad, the Corps is the fullest expression of who he is. For Nate, the Corps was a proving ground for his values of leadership, integrity, and compassionate care, that ultimately point him past it.
To the Brad/Nate shipper in me, this reading is exactly what binds them together. Their dynamic was forged in intensity, they are two people who understood each other completely inside one specific, extreme context. For two people adept at managing men and superiors, they carry an air of melancholy that only the other sees. And despite their difference in rank, the show often has them blurring the lines to communicate via gaze and words. In Iraq, they build a bond, permanent and singular, that civilian life cannot touch or diminish and which cannot be replicated in ordinary life.
Brad's respect for Nate's judgment and Nate's reliance on Brad's candour is the emotional throughline that sustained them in Iraq and that doesn't need combat to function. It only needs two people who still tell each other the truth, who each recognizes the other's version of integrity as real even when it points to a different direction than his own.
