Great Britain entered the First World War due to the Germans violation of the Treaty of London (1839) in defence of the Belgium Neutrality.
The tension between Great Britain and Germany had been rising for some time due to the determination of the Germans to rival the United Kingdom’s dominance in its Navy.
The Germans invaded Belgium on the way through to France in 1914 with the British protesting against Germany regarding their obligations of the treaty.
German Foreign Minister Theobald von Bethmann Hollweg
referred to the treaty as a " Mere scrap of paper" and could not believe Britain was entering the War.
With the treaty being almost 100 years old the Germans did not expect the British to stand by their pledge.
The British Army had sustained substantial losses in the Horror and July 1st 1916 in The Battle of the Somme and regarded as the worst disaster in the History of the British Army.
But as the war unfolded this paved a new way for a new type of warfare and an inspiration for new technology and weapons in modern War that led to victory which was finally achieved in 1918.
The Royal Flying Corps
The British Army
The Royal Navy