The central character is an English rifleman called Dodd- a sharpshooter- who has got himself stranded behind enemy lines and must somehow make his way back to his mates. Dodd is a professional fighting man; he's been at it since his late teens and it's a better life than he could have hoped for as a peasant farmer in England. He regards the native Portuguese as ever so-slightly less than human and is very good at what he does- which is killing Frenchman- or whoever else the vagaries of international politics direct him to shoot at. He is business-like, hardened but not cruel, bad-tempered but who can blame him- and if he loves anything it's his regiment.
We also get chapters from the point of view of the Frenchmen whom Dodd is killing. They're a bunch of copains, poor bloody infantry, pushed around, starved, ignorant, the lowest of the soldierly low, lacking Dodd's elite skills, just there to make up numbers on the killing fields. They're not such bad sorts considering they're the scum of the earth.
Forester- writing in the mid 1930s- views the Peninsular from the far side of the experience of '14-'18- as he makes explicit every now and again with references to the "future" refinements of barbed wire and poison gas. You think the Great War was uniquely horrible? Not so, he says. War was always just about the stupidest thing men do.