On this day in 1916, 150,000 British troops emerged from their trenches to attack the German lines near the River Somme in northern France. Around 20,000 of those men would not live to see night fall.

July 1, 1916 was the worst day in the history of the British Army - and it didn't end there.

As the archive footage in the video above recalls, the Battle of the Somme would rage until November, transmogrifying into one of the bloodiest battles in the history of human conflict. Those five months of barbarous fighting would move the Allied front a mere six miles to the east at the cost of more than one million lives.

A great deal has been made of the strategic decisions that preceded the slaughter, with the academic consensus not falling far short of the old adage concerning lions being led by donkeys.

British infantrymen occupy a shallow trench in a ruined landscape before an advance during the Battle of the Somme.

Much of the blame has fallen on the shoulders of General Douglas Haig, the British commander of the portion of the Western Front which included the site of the Battle of the Somme. But he was not alone.

With the French taking severe losses at Verdun to the east of Paris, the Allied High Command decided to draw Germans troops away from the area by staging a concerted attack on enemy lines to the east of Amiens, around 100 miles north of Paris.

[Read more: June 6, 1944: D-Day's Normandy landings mark the start of liberation of Nazi-occupied Europe]

The Allies opened the offensive with a week-long pounding of the German line, firing over one and a half million shells at what was believed to have been a relatively soft section of the Western Front.

Once the enemy positions had been destroyed, British soldiers would be clear to 'go over the top' and take the German trenches, pausing only to wave the horses of the cavalry through the breaches.

But the seven-day bombardment merely served as a warning to the Germans. Their generals immediately saw it for what it was: a prelude to a big push. They simply moved their troops to underground bunkers and there they waited.

A British Grenadier Guardsman keeps watch on 'No-Man's land' as his comrades sleep in a captured German trench at Ovillers, near Albert, during the Battle of the Somme in 1916.

When the artillery guns fell silence on the morning of July 1, the Germans moved into position and set their sights on Allied trenches.

What soon emerged - walking slowly towards them across No Man's Land - was not an army of professional soldiers. This was 'Lord Kitchener's Army', volunteers mustered following his call for recruits at the start of the war and made up of 'Pals' battalions, recruited from the same places and occupations.

They never stood a chance.

Although a few units managed to reach German trenches, they could not exploit their gains and were driven back. By the end of the day, the British had suffered almost 60,000 casualties, including 19,240 dead.

Following the death of around 60% of his officers in just a single day of fighting, General Haig was forced to concede that the planned breakthrough had failed.

Months of bloody stalemate ensued, with British and French forces probing other areas of the German lines with very limited success.

British troops coming out of the trenches at Guillemont in September 1916.

The battle of attrition petered out as winter closed in, with the Allies claiming victory on account of the relief of Verdun and a slight shift in the position of the Western Front.

But the cost was exorbitant: the British suffered around 420,000 casualties, the French 195,000 and the Germans around 650,000.

Despite the ultimate victory which followed in its bloody wake, the Battle of the Somme has come to symbolised the futility of human conflict and the dark horrors of mechanised warfare.