World war one started because of the assassination of Archduke Franz Ferdinand of Austria-Hungary. ... When Russia began to mobilize due to its alliance with Serbia, Germany declared war on Russia.
The simplest answer is that the immediate cause was the assassination of Franz Ferdinand, the archduke of Austria-Hungary. His death at the hands of Gavrilo Princip – a Serbian nationalist with ties to the secretive military group known as the Black Hand – propelled the major European military powers towards war.
