Europe in 1914 was an armed camp; its politics dominated by two rival alliances. The creation of a unified Germany in 1871 had disturbed the old 'balance of power' in Europe.
Fear of Germany encouraged France and Russia to form an alliance in 1894. This pushed Germany into closer alliance with its neighbour, the Austro-Hungarian Empire. The members of these rival power blocs maintained mass armies through compulsory military service. Rapid developments in military technology forced them to spend huge sums on these armies.
During the 1900s, a dangerous rift arose between Russia and Austria-Hungary, who had conflicting ambitions in South Eastern Europe. Austria-Hungary's desire to crush Serbia, and Russia's support for the latter during the crisis of 1914, were motivated by fear that they would lose their status as 'Great Powers' if they backed down.
Britain's policy was to maintain a balance of power in Europe. Germany's growing strength and manifest pursuit of 'world power' status persuaded Britain to align with its traditional rivals: France in 1904 and Russia in 1907. This connected Britain, France and Russia in the 'Triple Entente' and stoked German fears of 'encirclement'. German nationalists viewed Britain as a barrier to their global ambitions and German generals increasingly feared the growing military threat of Russia.
In August 1914, the military and political leadership of Germany concluded that war should risked 'now or never' if they were to achieve their vision of Germany's destiny. They planned to defeat France swiftly, before Russia could marshal its forces. The invasion of France and the violation of Belgian neutrality brought Britain into the war.