Catalyst to war was the assassination of heir to Habsburg throne, Franz Ferdinand, in Bosnia. Austria declared war on Serbia. Germany then declared war on Russia and France and invaded neutral Belgium. This brought the British into the war on the side of the French. Japan joined Britain as did Italy in 1915.
Military campaigns center on france and Belgium in W Europe.
First battle of Ypres prevented the Germans from reaching the ports. By end of 1914 static line of defence had been established from Belgian coast to Switzerland. Position of stalemate reached. 1916 allies launched offensive for the W front but stopped by Germans who attacked French at Verdun. To relieve situation Battle of the Somme was launched but proved indecisive.