"But ranged as infantry, (5)
This is the first hint we get in the poem that our speaker is talking about war. And it's not like he was up in an airplane, or working as a medic in some hospital tent. The guy was on the front lines, facing down men who probably looked a lot like him.
And staring face to face, (6)
At this point, we can probably see what's coming. He's staring down his supposed enemy, who—we know from the first stanza—could just have easily been his bestie. It's only a matter of time before this meeting comes to an end.
"I shot him dead because—
Because he was my foe, (9-10)
The hesitation, the repetition—they tell us that this dude doesn't quite buy what he's about to sell. He tells his listener that he shot the guy opposite on the battlefield because the guy was his foe. But isn't that begging the question? We mean, who says he was his foe? And why?
"Yes; quaint and curious war is! (17)
Remember, these are the speaker's words—not Hardy's. If he were talking here, he'd probably say something like, "Awful and atrocious war is!" Or "Heinous and pointless war is!" Still, though he's using these quirky words—we get the picture. War is not quaint and curious. It's cruel and capricious, thank you very much.
"Had he and I but met (1)
This desire for an alternative meeting between the men is our first clue that the speaker is attempting to take control of the events in his mind in order to change the outcome. Maybe he feels so guilty, he's literally wishing for this alternate reality to come true.
I shot at him and he at me (7)
The matter of fact language masks the speaker's guilt just as the understatement of the entire poem masks the traumatic event. He's basically saying, Yep, I shot the guy. What of it? while thinking, Good grief, I shot a guy.
"I shot him dead because— (9)
Hardy uses punctuation to imitate the speaker's confusion. The speaker literally stops himself midsentence because he realizes he has absolutely no clue how to finish the sentence in the first place. And if you've got no good reason to kill a man, well, then that means you're guilty. Harsh, but true.
Yes; quaint and curious war is! (17)
Well, that's two words for it. They're not the words Shmoop would choose, but they are the words you might choose if you were our speaker, trying to distance himself from what he's done on the battlefield.
By some old ancient inn, (2)
Not that you would be expected to know this, but it wasn't typical for middle-class or wealthy Englishmen to drink at inns like this. Ancient inns were more for the rough-around-the-edges types.
"But ranged as infantry, (5)
Wealthy, educated men in the military were often officers of one sort or another, and infantrymen were lower-class men hoping to distinguish themselves in battle. This was one way to rise in society, but it's also a strong indication of just how much class matters on the battlefield. After all, it's the working class infantry men who die in the greatest numbers—they're the ones on the front lines.
Was out of work—had sold his traps— (15)
The speaker is letting us know that he didn't have much choice but to join the army, because he had no job or money. Sure, he's saying it about the man he killed, but he might as well be sharing his own life story. It's clear this guy was not a born warrior.
You'd treat if met where any bar is,
Or help to half-a-crown. (19-20)
The speaker is obviously a generous guy, and probably a pretty good friend. Which makes the fact that he had to kill this other man an even bigger bummer. And the only reason he had to in the first place is that he couldn't afford to stay out of the war. Or at least, that's one way of looking at it.