Crowds, many waving boaters, cheer Britain's declaration of war on Germany in Trafalgar Square, London. (Photo by Hulton Archive/Getty Images)

Police in Sarajevo arrest a man after a failed assassination attempt on the life of Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire. The arrested man previously thought to be Gavrilo Princip, who succeeded in killing the Archduke the same day, is now thought to be one of his six co-conspirators Nedeljko Cabrinovic. The assassination was one of the causes of World War I. The moment was captured by amateur photographer Milos Oberajger. (Photo by Milos Oberajger/Topical Press Agency/Getty Images)

Exuberance on Unter den Linden in Berlin, following the declaration of war, 4th August 1914. A photograph of Kaiser Wilhelm II is visible in the background. (Photo by General Photographic Agency/Hulton Archive/Getty Images)

Russian soldiers stoping a lorryload of deserters. (Photo by Hulton Archive/Getty Images)

Russian soldiers outside a marconi wireless station. (Photo by Topical Press Agency/Getty Images)

Mobilized Austrian troops and officers bid farewell to their wives and sweethearts. (Photo by Hulton Archive/Getty Images)

Belgian soldiers driving a Minerva armoured car during the First World War. (Photo by Hulton Archive/Getty Images)

A recruitment drive during the First World War at Trafalgar Square, London. (Photo by Central Press/Getty Images)

Two men conscripted to the British Army undergoing a medical check-up at Marylebone Grammar School, London. (Photo by Central Press/Getty Images)

French officers dining in style in a trench near the front line. (Photo by Hulton Archive/Getty Images)

Two army deserters are threatened by a Russian soldier with raised rifle and bayonet. (Photo by Hulton Archive/Getty Images)

British tommies relaxing and having wounds treated in an underground forward dressing station by the Menin Road in France. (Photo by Three Lions/Getty Images)

German sharpshooters move to a position near the front line, during the fighting near the Aisne River, 1914. (Photo by Keystone/Getty Images)

Injured soldiers rest in a cave in France. (Photo by Hulton Archive/Getty Images)

Wounded British and Belgium soldiers retreating from Mons during World War I. (Photo by Three Lions/Getty Images)

British soldiers, newly arrived in France preparing to go to the lines. (Photo by Three Lions/Getty Images)

French troops waiting to depart for the Belgian front. (Photo by Three Lions/Getty Images)

German infantry on manoeuvres in preparation for war. (Photo by Topical Press Agency/Getty Images)

Members of the 15th London Regiment receive their pay outside Somerset House, soon after mobilisation. (Photo by Hulton Archive/Getty Images)

German reservists going home to say goodbye to their families, before leaving to go to the front. The children with them are wearing their father's helmets. (Photo by Hulton Archive/Getty Images)