Long poems about battles have never been my thing. When I studied Anglo-Saxon at university, I remember complaining that whenever I wasn't sure of a word, it turned out to mean "spear". The number of words meaning spear seemed infinite. Perhaps I had Simon Armitage's The Death of King Arthur coming to me.

I approached the book with caution because even though Armitage's Sir Gawain and the Green Knight was masterly, it was a translation of one of the greatest Middle English poems ever written. This is not Malory's Le Morte d'Arthur but an anonymous, alliterative 4,000-line poem written in 1400. It seemed possible it would turn out to be a blunt spear.

The theme of the poem, Armitage explains in his introduction, is "the ever-topical matter of Britain's relationship with continental Europe, and the no less relevant subject of its military interests overseas". Hopes are not raised when he describes the translator's challenges. These include characters created for "alliterative convenience" and moments where knights killed in one section are mysteriously back on their feet in another. He forewarns us of repetitive imagery and tells how he has tried to preserve alliteration over several lines like a "knowingly extravagant riff". He also says that, unlike the original, he has opted for one uncontroversial tense in which to tell the story: the past. But he is never less than admiring and forgiving of the anonymous medieval scribe who laboured by candlelight.

With Armitage's translation, there is nothing to forgive. He has a miraculous ability to make the past fresh, moving and urgent, not allowing legend to create distance. And while you do need to be in friskily bellicose spirits to read an almost non-stop account of battles (spears aplenty, with livers, guts and other body parts glistening on their points), this is an extraordinary work: fighting verse. It is the versifier's job to make order out of chaos. Whatever happens on the battlefield, the poet must take the strain. And there are occasions when this could easily have been a close shave. Hundreds of horsemen gallop against the grain of the verse but Armitage's sanity and grace spur the narrative on.

There are extraordinary set pieces, too  mainly concerning Arthur's dream life  a fight between a dragon and a bear, a nightmare about Fortune's wheel. There is also a tremendous account of the Mont St Michel monster , first encountered snacking on a human thigh and wearing a gown bordered by the beards of kings: "made to measure/ sewn by specialist Spanish maids". Armitage approaches his alliterative task with unforced panache. "The startled glutton glared gruesomely" reads like a grinding of teeth.

There are beautiful, incidental glimpses of landscape, comparable to those one sometimes spots in medieval painting ("shrubs shone under shimmering trees", or 5,000 riders "surging through white water"). There is resplendent sartorial detail: "His gauntlets shone with gold and were edged at the hem / with seed pearls".And the ending is extraordinarily moving about the death of Gawain (completion for Armitage?) and Arthur's abject finale.

But what I loved most were the rare moments of peace, such as this account of knights taking a break:

"lords leaning and lying on their shining shields,
and their love of birdsong brought their laughter aloft
of the lark and the linnet and their lovely tunes.
And some slipped into sleep, soothed by these creatures
singing of the season in the sun- kissed woods,
lulled by the music which murmured through the land."

Simon Armitage has already translated one of the great pre-Renaissance English poems, Sir Gawain and the Green Knight (2007). Lines from Gawain  "Sumwhyle with wodwos he werrez, and with wolves als" and so on  formed the epigraph to Ted Hughes's 1967 collection Wodwo, a book that showed the then future poet laureate in training for his bloodstained epic Crow ( 1970). Crow is a sort of nightmare beast fable taking place as much in the back of the mind as in the outer world, with the supernatural gradually giving way to Freud. To some degree Armitage's latest work, a translation of the anonymous early 15th-century epic Alliterative Morte Arthure (not to be confused with Malory's prose Le Morte d'Arthur), works in parallel to Crow, though its concern, as befits a more secular poet such as Armitage, is with realpolitik rather than psychology. Not for the first time, Armitage seems to be fruitfully shadowing the predecessor whose work first inspired him.

Arthur is certainly several parts beast. His manners, like the narrator of Hughes's famous early poem "Hawk Roosting", often consist of "tearing off heads". When ambassadors from Lucius Iberius, emperor of Rome, arrive at Arthur's Christmas court in Carlisle (this is a northern poem) demanding fealty from the British king and commanding him and the Round Table knights to appear in Rome, it looks as though they might be leaving with their heads under their arms like the Green Knight in the poem next door. But Arthur treats them with terrifying courtesy before announcing his plans to invade Europe.

There's no room for the D-Day secrecy of Operation Overlord here:

Then will I move through the mountains and march on his heartlands,
to marvellous Milan to wallop down its walls.
In Lorraine and Lombardy I will leave not one man
who lives by his laws or is loyal to his cause,
then turn into Tuscany at a time that suits me
and ransack wide regions with my riotous knights.

The present-day resonance of this is hard to miss, with the Roman empire as the EU demanding a renunciation of British sovereignty. The poem could have been written for Eurosceptic MPs to dream over. If they are in a sense the officer class, Arthur's forces invade Europe with an indiscriminate savagery whose tribal echo can be found among their troops  the bullet-headed hordes of football thugs travelling abroad nowadays under the flag of St George. A ruck's a ruck, wherever it may be.

Armitage handles the alliterative verse with great energy and verve, attaining the momentum of a siege-tower falling off a cliff, and relishing the opportunity for comic boastfulness and gluttonous, bloodthirsty comedy. Before setting out for Rome, Arthur tackles the cannibal monster of Mont Saint-Michel, who "was bulky as a sea-pig with a brawny body, / and each quivering lump of those loathsome lips / writhed and rolled with the wrath of a wolf's head". With a generation of children raised on Horrible Histories, Armitage's version might do for alliteration what Eliot's Practical Cats once did for rhythm.

When battle is joined near the river Seine, it goes on for pages, in what Armitage might describe as a delirium of disembowelment. From time to time it's hard to know who's doing what to whom, except that it's grotesque and fatal. So great is the slaughter that it's surprising the continent is not depopulated. We seem to be reading about the hundred years' war, which was contemporary with the poem's composition, but also unavoidably looking forward to the catastrophes of the thirty years' war and the first world war. In victory, Arthur thanks "Christ and his cherished mother", and in Christ's name invites his forces to help themselves to the treasures of Toulouse. The emperor responds by dispatching a host of Saracens, and after a ritual exchange of insults  "truss up your trumpets and trifle no longer"  it all kicks off again, and again ("fifty thousand folk were quickly felled").

When Arthur finally kills Emperor Lucius it merely marks a pause in the larger conflict. Seemingly invulnerable, as great as Alexander, the king looks down on Lombardy from the Alps, declaring: "I am lord of this lovely land, I believe", in hubris forgetting his debt to the Almighty. He carries on "toppling towers and tormenting the locals", before Rome surrenders and he turns to contemplate a new crusade. That night he dreams of Lady Fortune, who tells him: "all you love you shall lose, and your life as well. / You have loitered in privilege and pleasure too long." Her resemblance to Chancellor Merkel is purely accidental.

Repent and build abbeys, says Arthur's adviser. Turn home to confront Mordred, the enemy within. You have wasted too much time on foreign policy. Remember you must die. And off Arthur gallops, back over the border between nation and myth, well served by Simon Armitage. Imaginatively, The Death of King Arthur isn't a patch on Gawain, but it certainly rips and roars and ravishes in handling one idea of the Matter of Britain. It would also make a great computer game  or perhaps a new Frank Miller movie to follow 300, even though it lacks Miller's humane liberal irony.

 Sean O'Brien's November is published by Picador.