His brain engorged with meaningless fury, Patrick Hennessey pins a slack-faced civilian by the gullet to the wall of a nightclub yelling: “I’m a f****ing killer!” It is a great scene. Not least because it doesn’t happen.

Hennessey, then a captain in the Grenadier Guards, is back from a six-month tour in Helmand, southern Afghanistan, where he was in charge of a team “mentoring” Afghan government troops, and fighting and killing and getting blown up and getting mangled and laughing and shouting and turning into warriors and loving it.

Back here, in the unreal world, life is boring, and silly, and the people living it are sillier. The politicians who ordered the two wars that Hennessey has fought in are idiots. The senior officers, especially in the Royal Navy and RAF, who insist on buying huge ships and useless warplanes because they are planning for “a war” rather than “the war(s)” that we are already fighting, are self-serving bureaucrats with no combat experience. And the media who whine about it all on behalf of soldiers on the front line just an interfering pain in the backside.

Hennessey is 25, a Balliol man with an English degree. He’s shot people dead in the heat of battle. And he’s noted, with abstract interest, how a Taleban fighter whom he killed with a borrowed sniper rifle folded and flopped out of a tree. His generation owns the war in Helmand. There has been nothing like it since Korea. And this is why he is sitting silent and scowling in a London nightclub fantasising about throttling some civvy who can never begin to understand the maddening heat, the exuberant glory of combat, and why he feels so empty now.

Not, that is, unless they read this harrowing and frequently funny book that sparkles with wit, wisdom and boyish glee.

Many, probably all, men have had fantasies of wartime heroics. But few would admit that they have deliberately put themselves, like Hennessey, in a position in which heroism is a necessity — the survival mechanism of any successful army. Still fewer would have the mature honesty to say afterwards: “We wanted documents signed in black and white and glinting metal forged with our names to shout to the rafters that what we had done was not wrong, not bad, but glorious and heroic, and we weren’t sick to feel that it had all been such f***ing good fun.”

Soldiers don’t grow old, nor the years condemn them, because they simply don’t grow up. War is what boys most want to do until they realise that it’s not, really, a game. In Iraq Hennessy and some friends start the reading club of the title to escape the grim boredom of a campaign that had run into the sand. But in Afghanistan he finds that there is no time for such luxuriating. Instead he records the gonzo kaleidoscope of his experiences in journals and e-mails home that he seamlessly sews into the pages of this book. Hennessey is an exceptional talent and was, in his own words, one of the “Army’s Bright Young Things”, highly educated, yearning for adventure and glory. Without them we would not have an army worth the name.

But one has to wonder at the wisdom of a government that has sent young men to shed blood on foreign fields while simultaneously, through cuts in personnel and equipment, crippling their ability to prevail. Worse, perhaps, is the present Government’s spectacular inability to comprehend or respect an army that it has been so quick to send to fight. Lord Mandelson once famously described Trooping the Colour as “a lot of chinless wonders marching around Horse Guards Parade doing incomprehensible things with flags”.

Hennessey is one of those “chinless wonders”. One can only hope that soon the veterans of the Blair and Brown conflicts will go into politics and save future generations from the natural, magnificent and childish desire to test themselves in combat.

The Junior Officers’ Reading Club: Killing Time and Fighting Wars by Patrick Hennessey (Allen Lane, £16.99; Buy this book; 352pp)

Desperate Glory: At War in Helmand with Britain’s 16 Air Assault Brigade, by Sam Kiley, will be published by Bloomsbury in October

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