World War I
Map: WW I
The first World War erupted when the expansionist policy of Austria-Hungary conflicted with Serbian nationalism.

Why the War happened

The military alignment at the beginning of the war :
The Central Powers (Germany, Austria- Hungary, and Turkey) were opposed by the Allies (Russia, France, Great Britain, Belgium, and Serbia) joined by Japan, Italy, and the USA.

How the War started

The war which ended in November 1918 involved the mobilization of 65 million men; nearly nine million died and about 22 million were wounded. It is estimated that the loss of civilian life due to war exceded that suffered by the armies in the field.

The First World War was the bloodiest that had been fought up to that time, and caused losses both to victors and vanquished out of all proportions to the issues involved.

www link :
Encyclopaedia of World War I

Trenches on the Web an esoteric but comprehensive web site about the First World War.