World War I broke out by accident. No European government wanted a general war, but most of the European powers perferred to fight rather than back down in the face of diplomatic provocation from their rivals.

The spark which provided the excuse to set the armies marching was the assassination of the heir to the Austrian throne in the summer of 1914. Austria immediately accused Serbia of instigating the murderers and issued an ultimatum, whereupon Russia declared her support of Serbia. Once the Russians ordered general mobilization (and they 'had to' because they needed more time to move their forces to the frontiers then their enemies) Austrian, French, and German mobilization orders followed in quick successsion, each triggered by the other.

Mobilization plans quickly changed into war plans, because to halt part way through the deployment of military forces would simply create chaos; and chaos invited enemy attack - exactly what mobilization was supposed to prevent ! Civilian leadership was displaced by military leadership, and rival armies plunged across European frontiers 'according to plan'

Germany and Austria struck at France and Russia and Serbia; the German armies marching through Belgium, Britain immediately came into war on the side of Belgium, bringing in Japan, as her ally, and very soon Turkey followed on the German and Austrian side.

Germany's aim was to encircle Paris and defeat France within the first few weeks of the war, in order to concentrate later against Russia on the East. There was a tremendous rush of the Germans upon Paris and an invasion of East Prussia by the Russians. Both attacks were held and turned.

Then the power of the defensive developed; there was a rapid elaboration of trench warfare until the opposing armies lay entrenched in long lines across Europe, unable to make any advances without enormous losses. It became apparent that the progress of modern technical science had changed the nature of warfare, a change that the most carefully advanced war plans did not anticipate. No one had expected four years of stalemate and the brutal butchering of  industrial war.