After a pause, the old man began to tell the story:
‘There was once a mercenary captain who had fought for many years in the Northern Wars. He had been so long a soldier that no one remembered his real name, indeed he may have forgotten it himself, and was known to both comrades and enemies alike simply as The Captain.
I need not tell you of the Northern Wars, save to say that they were long and fierce – pitting brother against brother, father against son – and by the time peace came the land was little more than a ruin, with both victor and vanquished alike left in bloody tatters.
Even men like The Captain eventually grew weary of the slaughter - he himself was hard, hard as flint with chilled water where other men had blood.

 

Firstly, I'm never convinced by the old "he might even have forgotten his own name" thing. It's frankly very unlikely, and while everyone around/serving him might have called him "the Captain", you're dealing with the precise time at which armies were beginning to become structured and professional (in Europe, for the first time since the Roman Empire), I'm sure those who employed him would have found another name for him.

Secondly, the very opening line: "After a pause, the old man began to tell the story:" OK, so you're setting up a frame, a storyteller is telling the story. There's nothing wrong with doing this so long as the frame serves an important purpose; whether as counterpoint, whether illuminatory because the story-teller turns out to have been in the story, whatever... but an opening one-liner doesn't provide any context at all. Who's the old man? Who's he talking to? Where is he? Why has he just paused? If these are relevant, tell us. If they're not, drop the frame motif completely.

I'd read on, as it stands, but only because of my interest in the historical period.
 

The entire second paragraph is a bit cliched. "Brothers against brothers" and "victor and vanquished," particularly the last phrase which is a weaker (and less eloquent) version of "he had ice water in his veins"

The writing isn't that bad, but it doesn't grab me either. There is nothing here to set it apart...
 

And, naturally, the old man's narrative isn't particularly eloquent by modern standards. This is no doubt intentional, but since we don't know why we're listening to the old man in the first place, it doesn't work. We know exactly three things about the story teller. He's male, old, and full of cliches. If you want us to buy him as a character worth our attention, we need more information than that.

I used to forget my name all the time. If I'd switched to another before the age of, say...fifteen, I might have seriously forgotton my original name.
 

Sometimes examples illustrate best, and a good (not perfect, but good) example that immediately came to mind is Guy De Maupassant's "The Mad Woman," which happens to be an old story I like a lot. It's on Project Gutenberg (free public domain books! woo-hoo!) here: http://www.gutenberg.org/dirs/etext96/swgem10.txt

A few lines of the intro is below. Note how the story is personal to the narrator, that he has a reason for telling the story to begin with. And really, that first paragraph tells us all we need to know to get a picture of who is telling the story and why. We learn more about him later, but it's enough to get it going. Well, I think so. Don't necessarily take my word for it. Good luck!

THE MAD WOMAN

"I can tell you a terrible story about the Franco-Prussian war,"
Monsieur d'Endolin said to some friends assembled in the
smoking-room of Baron de Ravot's chateau. "You know my house in
the Faubourg de Cormeil, I was living there when the Prussians
came, and I had for a neighbor a kind of mad woman, who had lost
her senses in consequence of a series of misfortunes. At the age
of seven and twenty she had lost her father, her husband, and her
newly born child, all in the space of a month.

"When death has once entered into a house...
 

The second sentence in quotations is too long and would read easier broken into two sentences.

"...both victor and vanquished alike..." is redundant. You don't need "both" AND "alike."

I guess I'd rather start with knowing something about the character who will serve as your POV. Somebody is listening to this old man, and that's the person I want to know about. Otherwise I feel too removed from the scene. (I think someone already pointed this out...)

I wasn't enthusiastic about where this fragment would lead. I wish I could be specific as to why, but all I have is a general impression. I think most of my difficulty stems from the fact that it all feels, as previously noted, a little cliched. There's nothing really fresh here. I think your writing is successful in that I could follow it, there aren't any glaring grammatical problems that make it hard to read, and the old man's dialogue is convincing as dialogue. It just maybe isn't the best place to start, and there should be something to set it apart from all of the other stories about career military leaders.
 

If your old man is "The Captain" mentioned in the story, this forecloses some of the options used in The Mad Woman, and so you'll have to concentrate on the POV's reasons for finding the old man intersting in the first place. A setting isn't a bad idea at all, either.