Vilnius, venerable capital of Lithuania, is sometimes called 'the city built on human bones'. It stands in the main Berlin to Moscow corridor, which for over 200 years has been the battlefields of the armies of Napoleon, the Tsars of Russia, Hitler and Stalin, as well as Poles and Prussians - hence its sinister description.

Early in 2002, while bulldozing some ugly Soviet barracks on the outskirts of Vilnius, municipal workers uncovered a mass grave. Thousands of skeletons were discovered there, laid out neatly in layers. Where did these bones come from? Were they those of Jews, massacred by the Nazis? No. For here's a metal button, with '61' stamped on it. Here's another, stamped '29'. And here's a patch of an ancient uniform, once blue. Also to be seen is a gold 20-franc coin from Napoleonic times, and a 'shako' (a French infantryman's helmet), squashed flat.

The drivers of the bulldozers stopped in their work. This was news - archaeological news - and these were the remains of some of the men that Napoleon had led into Russia in his pursuit of world supremacy in 1812.

By 1812 Napoleon had conquered the whole of continental Europe - from southern Italy to the Baltic, from Portugal to Poland. England herself he couldn't get at, not after the Battle of Trafalgar of 1805, when Lord Nelson had defeated the combined French and Spanish navies. Despite this, Napoleon hoped to undermine the economic superiority of Britain, by banning trade with her and excluding the products of the 'nation of shopkeepers' from European markets.

In 1807 the Tsar of Russia, defeated for the second time, had agreed not to trade with the British, but harsh economic reality spoke louder than treaties, and Russia continued to trade despite the ban. In response, on Midsummer Day in 1812, Napoleon crossed the River Niemen into what was then the Russian province of Lithuania, in a bid to conquer Russia with the biggest, most spectacular army Europe had ever raised. This army consisted of almost half a million men, only half of them French. The rest were drawn from Napoleon's European empire, the result of his conquests over many countries. Some of these, including Holland, for example, he incorporated, along with their armies, into France. In this way he had an almost inexhaustible supply of soldiers.

Now Napoleon had ten army corps, against the Russian Tsar's two. After a 'good battle' (as he called it) with his 'brother the Tsar', to bring him back into the fold, he planned - perhaps - to march their combined armies to India, and strangle the supplies of British gold that had been financing successive coalitions against France. The entire Russian campaign, in fact, was actually aimed at Britain.

Nothing went as planned. There was no battle in Lithuania - where the French leader had hoped to start his campaign. The Russian army simply withdrew. This made it possible, four days later, for Captain Victor Dupuy of the French 7th Hussars to gallop into Vilnius, at the head of the invading army. Eyewitness accounts describe the scene:

..the most joyous acclamations. The ladies in their party dresses were throwing down flowers and biscuits to us from the windows.'

'…all the windows were filled with wildly enthusiastic ladies. Every hand seemed to be waving a handkerchief.'

The faster the Russians withdrew, the further Napoleon was dragged into Russia. Tens of thousands of soldiers, many of them very young French and allied soldiers, died of exhaustion, thirst or starvation in its summer heats ('worse than anything we'd known in Egypt').

Then at Borodino, a week's march from Moscow, the French and Russian armies, by now about equally matched, fought to a sanguinary standoff. Napoleon was undeterred, however, and marched on to the almost deserted Moscow, which the next day was sent up in flames - burnt down by its Russian governor. The French leader hung around for eight weeks, arrogantly waiting for the Tsar - who was in St Petersburg - to make peace. The Tsar, however, was by now in no mood for negotiation. 'My campaign, led by General Winter, is just beginning', he said. 'There can be no peace with Napoleon.' Napoleon, laden with booty, eventually set off to lead his army back to France, just as winter was approaching.

The snow came down, men froze, and horses starved. The last lap of the almost two-month trek back to Vilnius, was the worst of all. The soldiers barely managed the crossing of the Berezina River - over two frail bridges - and there were perhaps as only as few as 50,000 half-stunned survivors of the Grand Army who, harried by Cossacks, tottered on through icy temperatures towards the town. Man after man 'did a bear', tumbling with his haggard face downwards into a snow-filled ditch, never to rise again.

On the icy morning of 9 December 1812, outside Vilnius's deep vaulted gate, Victor Dupuy (now a colonel) had to be prevented by his few surviving comrades from sitting down and dying, 'overcome by lassitude and drowsiness, gripped by the frost'. Another (Belgian) officer, Francois Dumonceau, had to lead his horse over:

'a veritable moving mountain, more than 2 metres deep, of dead and dying, pushing, shoving, hemmed in on all sides, at each step risking being thrown down by the convulsive spasms of those we were trampling underfoot.'

Some of these unfortunates certainly ended up in the mass grave discovered in 2002. And there are sure to be other graves, too, as yet undiscovered. Probably as many as a half of the starving survivors who had managed to reach Vilnius died once they got there. They may have over-eaten, in their desperation to assuage their hunger, or drunk themselves silly. Many had frost-bitten noses, toes or fingers, which turned gangrenous. Some died of exhaustion or cold almost on arrival. As for lodgings, 'the stronger drove out the weaker', so that many a soldier, especially those with no Moscow gold to pay with, froze to death on an inhospitable doorstep.

Others again simply refused to go on, or were captured by the Cossacks - who had harried them throughout their retreat, and had starved the army to death by keeping it to one narrow highway. The prisoners were driven naked all the way back again into Russia.

Although Vilnius's 17 typhus-ridden monasteries had already been turned into makeshift hospitals, they lacked food or medicines, and many thousands of men died in them. On entering one such monastery-hospital, General Sir Robert Wilson (Britain's emissary to the Tsar), who had arrived in Vilnius with the Russians, saw thousands of bodies 'strewed [sic] about in every part… all the broken windows and walls were stuffed with feet, legs, arms, hands …to fit the apertures, and keep out the air from the yet living.'

In another account, Count Rochechouart, a French aristocrat in the Tsar's service, tells how he did his best to stop Russian soldiers flinging the 'yet living' out of upstairs windows to make room for their own wounded. And yet another description comes from the German writer Ernst Moritz Arndt, who arrived in January 1813 to see the frozen corpses piled up three-storeys high, and to hear them 'rattling' in the streets as sleighs went about collecting them.

Some he saw 'flung into the Vilia river' to float down to the Niemen and out into the Baltic where, he said, 'they'll make a meagre diet for the fishes.' Meanwhile, the glamorous French cavalry leader 'King' Joachim Murat (Napoleon's brother-in-law whom he had made King of Naples) was left in command at Vilnius. He, however, simply panicked and fled, declaring 'I'm not going to be trapped in this piss-pot'.

The stunned, frozen and starving spectres who had managed to stagger to Vilnius, many of them to end their days there, had come from all over French-occupied Europe. Eventually, at most some 20,000 soldiers - of the 400,000 who'd marched into Russia at midsummer - finally recrossed the Niemen into Poland. They were meant to rejoin Napoleon, but he'd already gone ahead to Paris to give the news of the catastrophe, and to raise new armies. Men could easily be replaced, but not horses. Tens of thousands of soldiers had died in Russia, but it was because of his lack of cavalry that Napoleon was eventually defeated by Austria, Prussia, Sweden and Russia, in 1813.

Successive occupiers - Russians, Prussians, Poles, Nazis, Soviets - have tried to stamp out Lithuanians' memories of their national history. And indeed many of today's Lithuanians, newly coopted into the EU, in fact know absolutely nothing about the horrific events at Vilnius in December 1812. The discovery of the mass grave has made Lithuanians aware of what happened, and hopefully this knowledge will help ensure that such things never occur again.

Now, 183 years later, the splendid museum in Vilnius displays many objects relating to the Napoleonic adventure. What's this button, made of an alloy of copper and tin, stamped '61'? It comes from a blue uniform jacket, almost certainly that of a Dutchman. For the 61st Line Regiment was made up largely of (mostly unwilling) conscripts from the Netherlands

This helmet plaque, with the remains of a tricolour cockade and an imperial eagle upon it, must be that of someone who fought at Borodino and got to Moscow - only to collapse and die in Vilnius. And this sleeve-button, stamped with a '29'? A relic of some recruit in Loison's ill-fated reserve division, which in December 1812 was brought up in far sub-zero temperatures, and in summer clothing, to save anything that could be saved of the doomed wreckage of Napoleon's army. Unfortunately these young men died too, almost to a man.

The Lithuanians have allowed some bones and teeth from the recovered bodies to be brought to the UK for lead isotope testing. The earliest water we drink as children leaves an indelible fingerprint in our teeth and bones, which means that isotope testing can tell Dr Mike Richards - the British pathologist involved in the analysis - what part of Europe the owner of a particular bone or tooth stemmed from, whether from the Po, the Elbe, or the Seine. The tests have also helped determine the cause of death of some of the victims, as well as what illnesses they suffered from. None of them seem to have died in battle.

This summer there's to be a solemn reinterment ceremony in Vilnius. It will be attended by ambassadors from every European Union country that once contributed, whether they liked it or not, to Napoleon's Grand Army of 1812.

Napoleon's Lost Army: The Soldiers Who Fell