Archduke Franz Ferdinand, of Austria-Hungary was murdered by a Serbian born anarchist, in Sarajevo Bosnia June 1914. The Archduke was the heir to the Austria-Hungary empire, and Serbia, was blamed for the murder.

Austria-Hungary first made a list of demands that it presented to Serbia. When the demands were not met in full, Austria-Hungary declared war on Serbia.

Russia which had an alliance with Serbia, mobilized it's troops and prepared to defend Serbia. Germany mobilized to defend it's ally Austria-Hungary, from Russian intervention. This was how the great war began, it was a regional issue that spiraled out of control.

More Info: en.wikipedia.org