An accurate death toll over the course of the Battle of the Somme is an impossibility. But it is thought that as many as a million men were killed in those 20 weeks.
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In early 1917, the Germans fell back to the safety of the Hindenburg line - negating all the gains for which the Allies had paid such a high price.
The Battle of the Somme saw British improvements in trench warfare. The creeping barrage was improved; tanks were used for the first time; intelligence got better.Copyright: Getty Images
The Battle of the Somme would continue for almost five months. The British lines moved forward by about 4.5 miles (7km).Copyright: Getty Images
As dusk descends on the Somme, it becomes apparent that the British army has suffered its bloodiest day. In all, the first day saw 57,470 British casualties - 19,240 dead. In comparison the German army had around 6,000 casualties.Copyright: Getty Images
The 36th Ulster Division finishes the first day of the Battle of the Somme right back where they started. But with more than 5,000 casualties - around 2,069 of them lying dead.Copyright: Getty Images
The battle rages on into the evening. The first day is yet not over but the casualties count is rising at an alarming rate.Copyright: Getty images
On the way back towards the British front lines, they meet the reinforcements they were hoping for about seven hours earlier.Copyright: Getty Images
The Germans are exhausted and the 36th manages its retreat in an orderly fashion.Copyright: Getty Images
The Ulster men fall back to the initial German front line. They brace themselves for an onslaught.Copyright: Getty Images
The order is given to relinquish the Schawben Redoubt.Copyright: Getty Images
Fewer men and fewer bullets means the Ulster Division's defence is becoming increasingly desperate. The Germans just keep coming.Copyright: Getty Images
Another determined German counter-attack. It's coming from three sides.Copyright: Getty Images
So far, the Germans haven't succeeded in breaking through the Ulstermen. But it's only a matter of time.Copyright: Getty Images
With scant possibility of reinforcements any time soon, the 36th Division have to attempt to hold on until dark.Copyright: Getty Images
The German counter-attacks just keep coming.Copyright: Getty Images
The counter attack from the Germans is fierce coming from the direction of the village of Grandcourt.Copyright: Getty Images
Communication with the Ulstermen through No Man's Land is nigh on impossible.
The men from the 36th who are at Schwaben are effectively cut off from their nearest comrades. No-Man's Land isn't an option for the Ulster Division at the minute - the German machine gun fire from Thiepval is almost constant.Copyright: Getty Images
The Germans want the Schwaben Redoubt back and start their counter attacks to take it.Copyright: Getty Images