The proximate cause for the war was the June 28th, 1914 assassination of Archduke Franz Ferdinand of Austria, heir to the Austro-Hungarian throne, by a Serbian nationalist, Gavrilo Princip. Gavrilo was a 19 year old boy who was a member of a nationalist group called the Black Hand Movement. Austria-Hungary's resulting demands against the Kingdom of Serbia led to the activation of a series of alliances which within weeks saw all of the major European powers at war. As a consequence of the global empires of many European nations, the war soon spread worldwide. good answer for the paragraph above also Germany were jealous of Britain and her allies as they had more countries under there belt such as Australia and India (colonies) Germany in turn went on austria's side and russia went on Serbia side as Serbia was also a slavok nation.
